3d ic semiconductor device with memory

ABSTRACT

A 3D semiconductor device including: a first level including a single crystal layer, a plurality of first transistors and at least one metal layer, where the at least one metal layer overlays the single crystal layer and includes interconnects between the first transistors forming control circuits; a second level overlaying the at least one metal layer, the second level including a plurality of second transistors; a third level overlaying the second level, the third level including a plurality of third transistors; and polysilicon pillars, where the second level includes a plurality of first memory cells, the first memory cells each including at least one of the second transistors, where the third level includes a plurality of second memory cells, the second memory cells each including at least one of the third transistors, where at least one of the second memory cells is at least partially atop of the control circuits.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/727,592, filed on Oct. 7, 2017; which is a continuation of U.S.patent application Ser. No. 15/351,389, filed on Nov. 14, 2016, now U.S.Pat. No. 9,799,761, issued on Oct. 24, 2017; which is a continuation ofU.S. patent application Ser. No. 14/506,160, filed on Oct. 3, 2014, nowU.S. Pat. No. 9,496,271, issued on Nov. 15, 2016; which is acontinuation of U.S. patent application Ser. No. 13/792,202, which wasfiled on Mar. 11, 2013, now U.S. Pat. No. 8,902,663, issued on Dec. 2,2014; the entire contents of all of the foregoing are incorporatedherein by reference.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention relates to the field of monolithic 3D integration tosemiconductor chips performing logic and/or memory functions.

2. Discussion of Background Art

Over the past 40 years, the microelectronic industry has seen a dramaticincrease in functionality and performance of Integrated Circuits (ICs).This has largely been due to the phenomenon of “scaling” i.e. componentsizes within ICs have been reduced (“scaled”) with every successivegeneration of technology. There are two main classes of components inComplementary Metal Oxide Semiconductor (CMOS) ICs, namely transistorsand wires. With “scaling”, transistor performance and density typicallyimprove and this has contributed to the previously-mentioned increasesin IC performance and functionality. However, wires (interconnects) thatconnect together transistors degrade in performance with “scaling”. Thesituation today is that wires dominate performance, functionality andpower consumption of ICs.

3D stacking of semiconductor chips is one avenue to tackle issues withwires. By arranging transistors in 3 dimensions instead of 2 dimensions(as was the case in the 1990s), one can place transistors in ICs closerto each other. This reduces wire lengths and keeps wiring delay low.However, there are many barriers to practical implementation of 3Dstacked chips. These include:

-   -   Constructing transistors in ICs typically require high        temperatures (higher than −700° C.) while wiring levels are        constructed at low temperatures (lower than −400° C.). Copper or        Aluminum wiring levels, in fact, can get damaged when exposed to        temperatures higher than 400° C. If one would like to arrange        transistors in 3 dimensions along with wires, it has the        challenge described below. For example, let us consider a 2        layer stack of transistors and wires i.e. Bottom Transistor        Layer, above it Bottom Wiring Layer, above it Top Transistor        Layer and above it Top Wiring Layer. When the Top Transistor        Layer is constructed using Temperatures higher than 700° C., it        can damage the Bottom Wiring Layer.    -   Due to the above mentioned problem with forming transistor        layers above wiring layers at temperatures lower than 400° C.,        the semiconductor industry has largely explored alternative        architectures for 3D stacking. In these alternative        architectures, Bottom Transistor Layers, Bottom Wiring Layers        and Contacts to the Top Layer are constructed on one silicon        wafer. Top Transistor Layers, Top Wiring Layers and Contacts to        the Bottom Layer are constructed on another silicon wafer. These        two wafers are bonded to each other and contacts are aligned,        bonded and connected to each other as well. Unfortunately, the        size of Contacts to the other Layer is large and the number of        these Contacts is small. In fact, prototypes of 3D stacked chips        today utilize as few as 10,000 connections between two layers,        compared to billions of connections within a layer. This low        connectivity between layers is because of two reasons: (i)        Landing pad size needs to be relatively large due to alignment        issues during wafer bonding. These could be due to many reasons,        including bowing of wafers to be bonded to each other, thermal        expansion differences between the two wafers, and lithographic        or placement misalignment. This misalignment between two wafers        limits the minimum contact landing pad area for electrical        connection between two layers; (ii) The contact size needs to be        relatively large. Forming contacts to another stacked wafer        typically involves having a Through-Silicon Via (TSV) on a chip.        Etching deep holes in silicon with small lateral dimensions and        filling them with metal to form TSVs is not easy. This places a        restriction on lateral dimensions of TSVs, which in turn impacts        TSV density and contact density to another stacked layer.        Therefore, connectivity between two wafers is limited.

It is highly desirable to circumvent these issues and build 3D stackedsemiconductor chips with a high-density of connections between layers.To achieve this goal, it is sufficient that one of three requirementsmust be met: (1) A technology to construct high-performance transistorswith processing temperatures below −400° C.; (2) A technology wherestandard transistors are fabricated in a pattern, which allows for highdensity connectivity despite the misalignment between the two bondedwafers; and (3) A chip architecture where process temperature increasebeyond 400° C. for the transistors in the top layer does not degrade thecharacteristics or reliability of the bottom transistors and wiringappreciably. This patent application describes approaches to addressoptions (1), (2) and (3) in the detailed description section. In therest of this section, some background art that has previously tried toaddress options (1), (2) and (3) will be described.

U.S. Pat. No. 7,052,941 from Sang-Yun Lee (“S-Y Lee”) describes methodsto construct vertical transistors above wiring layers at less than 400°C. In these single crystal Si transistors, current flow in thetransistor's channel region is in the vertical direction. Unfortunately,however, almost all semiconductor devices in the market today (logic,DRAM, flash memory) utilize horizontal (or planar) transistors due totheir many advantages, and it is difficult to convince the industry tomove to vertical transistor technology.

A paper from IBM at the Intl. Electron Devices Meeting in 2005 describesa method to construct transistors for the top stacked layer of a 2 chip3D stack on a separate wafer. This paper is “Enabling SOI-Based AssemblyTechnology for Three-Dimensional (3D) Integrated Circuits (ICs),” IEDMTech. Digest, p. 363 (2005) by A. W. Topol, D. C. La Tulipe, L. Shi, etal. (“Topol”). A process flow is utilized to transfer this toptransistor layer atop the bottom wiring and transistor layers attemperatures less than 400° C. Unfortunately, since transistors arefully formed prior to bonding, this scheme suffers from misalignmentissues. While Topol describes techniques to reduce misalignment errorsin the above paper, the techniques of Topol still suffer frommisalignment errors that limit vertical contact dimensions between twochips in the stack to >130 nm, and; hence, limits device density.

The textbook “Integrated Interconnect Technologies for 3D NanoelectronicSystems” by Bakir and Meindl (“Bakir”) describes a 3D stacked DRAMconcept with horizontal (i.e. planar) transistors. Silicon for stackedtransistors is produced using selective epitaxy technology or laserrecrystallization. Unfortunately, however, these technologies havehigher defect density compared to standard single crystal silicon and donot provide a mono-crystalline stacked layer or layers. This higherdefect density degrades transistor performance and device yield.

In the NAND flash memory industry, several organizations have attemptedto construct 3D stacked memory. These attempts predominantly usetransistors constructed with poly-Si or selective epi technology as wellas charge-trap concepts. References that describe these attempts to 3Dstacked memory include “Integrated Interconnect Technologies for 3DNanoelectronic Systems”, Artech House, 2009 by Bakir and Meindl(“Bakir”), “Bit Cost Scalable Technology with Punch and Plug Process forUltra High Density Flash Memory”, Symp. VLSI Technology Tech. Dig. pp.14-15, 2007 by H. Tanaka, M. Kido, K. Yahashi, et al. (“Tanaka”), “AHighly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash UsingJunction-Free Buried Channel BE-SONOS Device,” Symposium on VLSITechnology, 2010 by W. Kim, S. Choi, et al. (“W. Kim”), “A HighlyScalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash UsingJunction-Free Buried Channel BE-SONOS Device,” Symposium on VLSITechnology, 2010 by Hang-Ting Lue, et al. (“Lue”) and “Sub-50 nmDual-Gate Thin-Film Transistors for Monolithic 3-D Flash”, IEEE Trans.Elect. Dev., vol. 56, pp. 2703-2710, November 2009 by A. J. Walker(“Walker”). An architecture and technology that utilizes single crystalSilicon using epi growth is described in “A Stacked SONOS Technology, Upto 4 Levels and 6 nm Crystalline Nanowires, with Gate-All-Around orIndependent Gates (ΦFlash), Suitable for Full 3D Integration”,International Electron Devices Meeting, 2009 by A. Hubert, et al(“Hubert”). However, the approach described by Hubert has somechallenges including the use of difficult-to-manufacture nanowiretransistors, higher defect densities due to formation of Si and SiGelayers atop each other, high temperature processing for long times,difficult manufacturing, etc.

It is clear based on the background art mentioned above that inventionof novel technologies for 3D stacked chips will be useful.

SUMMARY

In one aspect, a 3D semiconductor device, the device comprising: a firstlevel comprising a single crystal layer, a plurality of firsttransistors and at least one metal layer, wherein said at least onemetal layer overlays said single crystal layer and comprisesinterconnects between said first transistors forming control circuits; asecond level overlaying said at least one metal layer, said second levelcomprising a plurality of second transistors; a third level overlayingsaid second level, said third level comprising a plurality of thirdtransistors; and polysilicon pillars, wherein at least one of saidsecond transistors comprises a first channel region and at least one ofsaid third transistors comprises a second channel region, wherein aportion of at least one of said polysilicon pillars is in direct contactwith said first channel region and said second channel region, whereinsaid second transistors are aligned to said first transistor with lessthan 150 nm alignment error, wherein said second level and said thirdlevel thickness is greater than 20 nm and less than 300 nm, wherein atleast one of said second transistors is self-aligned to at least one ofsaid third transistors, being processed following the same lithographystep, wherein said second level comprises a plurality of first memorycells, said first memory cells each comprising at least one of saidsecond transistors, wherein said third level comprises a plurality ofsecond memory cells, said second memory cells each comprising at leastone of said third transistors, wherein at least one of said secondmemory cells is at least partially atop of said control circuits, andwherein said control circuits are connected so to control at least oneof said second transistors and at least one of said third transistors.

In another aspect, a 3D semiconductor device, the device comprising: afirst level comprising a single crystal layer, a plurality of firsttransistors and at least one metal layer, wherein said at least onemetal layer overlays said single crystal layer and comprisesinterconnects between said first transistors forming control circuits; asecond level overlaying said at least one metal layer, said second levelcomprising a plurality of second transistors; a third level overlayingsaid second level, said third level comprising a plurality of thirdtransistors; and polysilicon pillars, wherein at least one of saidsecond transistors comprises a first channel region and at least one ofsaid third transistors comprises a second channel region, wherein aportion of at least one of said polysilicon pillars is in direct contactwith said first channel region and said second channel region, whereinsaid second transistors are aligned to said first transistor with lessthan 150 nm alignment error, wherein said second level comprises aplurality of first memory cells, said first memory cells each comprisingat least one of said second transistors, wherein said third levelcomprises a plurality of second memory cells, said second memory cellseach comprising at least one of said third transistors, and wherein saidcontrol circuits are connected so to control at least one of said secondtransistors and at least one of said third transistors; and a fourthlevel overlaying said third level, wherein said fourth level comprises acrystalline silicon layer.

In another aspect, a 3D semiconductor device, the comprising: a firstlevel comprising a single crystal layer, a plurality of firsttransistors and at least one metal layer, wherein said at least onemetal layer overlays said single crystal layer and comprisesinterconnects between said first transistors forming control circuits; asecond level overlaying said at least one metal layer, said second levelcomprising a plurality of second transistors; a third level overlayingsaid second level, said third level comprising a plurality of thirdtransistors; and polysilicon pillars, wherein at least one of saidsecond transistors comprises a first channel region and at least one ofsaid third transistors comprises a second channel region, wherein aportion of at least one of said polysilicon pillars is in direct contactwith said first channel region and said second channel region, whereinsaid second transistors are aligned to said first transistor with lessthan 30 nm alignment error, and wherein said control circuits areconnected so to control at least one of said second transistors and atleast one of said third transistors, and wherein said one metal layercomprises tungsten, wherein the height distance from said one metallayer to at least one of said first memory cells is less than onemicron, and wherein said single crystal layer has a thickness greaterthan ten microns.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustration of a floating-body DRAM;

FIGS. 2A-2M are drawing illustrations of a one-mask per layer 3Dfloating body DRAM;

FIGS. 3A-3K are drawing illustrations of a zero-mask per layer 3Dfloating body DRAM;

FIGS. 4A-4G are drawing illustrations of techniques to refresh floatingbody DRAM cells;

FIGS. 5A-5H are drawing illustrations of additional techniques torefresh floating body DRAM cells;

FIG. 6 is a drawing illustration of a floating body memory cell;

FIGS. 7A-7J are drawing illustrations of a 3D floating body DRAM withtwo stable states;

FIGS. 7K-7L are drawing illustrations of a 3D floating body DRAM dualport RAM with two stable states;

FIGS. 7M-7N are drawing illustrations of a 3D floating body DRAM dualport RAM with integrated select transistors with two stable states;

FIG. 8 is a drawing illustration of an architecture with a shared bitline access device;

FIG. 9 is a drawing illustration of two stable states memory back biasregion formed in the same mono-crystalline layer/region as the floatingbody channel;

FIGS. 10A-10J are drawing illustrations of a zero-mask per layer 3Dresistive memory with a junction-less transistor;

FIGS. 11A-11K are drawing illustrations of an alternative zero-mask perlayer 3D resistive memory;

FIGS. 12A-12L are drawing illustrations of a one-mask per layer 3Dresistive memory;

FIGS. 13A-13F are drawing illustrations of a two-mask per layer 3Dresistive memory;

FIGS. 14A-14F are drawing illustrations of a two-mask per layer 3Dcharge-trap memory;

FIGS. 15A-15G are drawing illustrations of a zero-mask per layer 3Dcharge-trap memory;

FIGS. 16A-16D are drawing illustrations of a fewer-masks per layer 3Dhorizontally-oriented charge-trap memory;

FIGS. 17A-17F are drawing illustrations of a two-mask per layer 3Dhorizontally-oriented floating-gate memory;

FIGS. 18A-18H are drawing illustrations of a one-mask per layer 3Dhorizontally-oriented floating-gate memory;

FIGS. 19A-19B are drawing illustrations of periphery on top of memorylayers;

FIGS. 20A-20E are drawing illustrations of a method to make high-aspectratio vias in 3D memory architectures;

FIGS. 21A-21E are drawing illustrations of polysilicon select devicesfor 3D memory and peripheral circuits at the bottom according to someembodiments of the current invention;

FIGS. 22A-22F are drawing illustrations of polysilicon select devicesfor 3D memory and peripheral circuits at the top according to someembodiments of the current invention; and

FIGS. 23A-23D are drawing illustrations of a monolithic 3D SRAMaccording to some embodiments of the current invention.

DETAILED DESCRIPTION

Embodiments of the invention are now described with reference to theindicated figures, it being appreciated that the figures illustrate thesubject matter not to scale or to measure. Many figures describe processflows for building devices. These process flows, which may be a sequenceof steps for building a device, may have many structures, numerals andlabels that may be common between two or more adjacent steps. In suchcases, some labels, numerals and structures used for a certain step'sfigure may have been described in previous steps' figures.

The entirety of U.S. Pat. Nos. 8,379,458, 8,273,610 and 8,803,206 areincorporated herein by reference.

Section 1: Monolithic 3D DRAM.

This Section describes some novel monolithic 3D Dynamic Random AccessMemories (DRAMs). Some embodiments of this invention may involvefloating body DRAM. Background information on floating body DRAM and itsoperation is given in “Floating Body RAM Technology and its Scalabilityto 32 nm Node and Beyond,” Electron Devices Meeting, 2006. IEDM '06.International, vol., no., pp. 1-4, 11-13 Dec. 2006 by T. Shino, N.Kusunoki, T. Higashi, et al., Overview and future challenges of floatingbody RAM (FBRAM) technology for 32 nm technology node and beyond,Solid-State Electronics, Volume 53, Issue 7, Papers Selected from the38th European Solid-State Device Research Conference—ESSDERC'08, July2009, Pages 676-683, ISSN 0038-1101, DOI: 10.1016/j.sse.2009.03.010 byTakeshi Hamamoto, Takashi Ohsawa, et al., “New Generation of Z-RAM,”Electron Devices Meeting, 2007. IEDM 2007. IEEE International, vol.,no., pp. 925-928, 10-12 Dec. 2007 by Okhonin, S.; Nagoga, M.; Carman, E,et al. The above publications are incorporated herein by reference.

As illustrated in FIG. 1 the fundamentals of operating a floating bodyDRAM are described. In order to store a ‘1’ bit, excess holes 102 mayexist in the floating body region 120 and change the threshold voltageof the memory cell transistor including source 104, gate 106, drain 108,floating body region 120, and buried oxide (BOX) 118. This is shown inFIG. 1(a). The ‘0’ bit may correspond to no charge being stored in thefloating body region 120 and may affect the threshold voltage of thememory cell transistor including source 110, gate 112, drain 114,floating body region 120, and buried oxide (BOX) 116. This is shown inFIG. 1(b). The difference in threshold voltage between the memory celltransistor depicted in FIG. 1(a) and FIG. 1(b) may manifest itself as achange in the drain current 134 of the transistor at a particular gatevoltage 136. This is described in FIG. 1(c). This current differential130 may be sensed by a sense amplifier circuit to differentiate between‘0‘ and’1’ states and thus function as a memory bit.

FIGS. 2A-M describe a flow to construct a horizontally-orientedmonolithic 3D DRAM (Dynamic Random Access Memory). This monolithic 3DDRAM utilizes the floating body effect and double-gate transistors. Onemask is utilized on a “per-memory-layer” basis for the monolithic 3DDRAM concept shown in FIG. 2A-M, while other masks are shared betweendifferent layers. The process flow may include several steps that occurin the following sequence.

Step (A): Peripheral circuits with tungsten, or conventionalaluminum/copper, wiring 202 are first constructed and above this a layerof silicon dioxide 204 is deposited. FIG. 2A illustrates the structureafter Step (A). This presented process flow suggests the use of tungstenwiring or other refractory metal so the following process steps couldinclude high temperature steps. These high temperature steps may includehigher than 400° C. such as 800° C. or even 900° C. for dopantactivation or damage annealing steps. Optical annealing and shieldingmay also be utilized, as described in the referenced patents. So theperipheral circuits activation could be delayed to those later hightemperature steps.

Step (B): FIG. 2B shows a drawing illustration after Step (B). A waferof p− Silicon 206 has an oxide layer 208 grown or deposited above it.Following this, hydrogen is implanted into the p− Silicon wafer at acertain depth indicated by 210. Alternatively, some other atomic speciessuch as Helium could be (co-)implanted. A portion of this hydrogenimplanted p− Silicon wafer 206 may form the top layer 212. The bottomlayer 214 may include the peripheral circuits 202 with oxide layer 204.The top layer 212 may be flipped and bonded to the bottom layer 214using oxide-to-oxide bonding.

Step (C): FIG. 2C illustrates the structure after Step (C). The stack oftop and bottom wafers after Step (B) may be cleaved at the hydrogenplane 210 using either an anneal or a sideways mechanical force or othermeans. A CMP (Chemical Mechanical Polishing) process may then beconducted. At the end of this step, a single-crystal transferred p−layer 207 exists atop the peripheral circuits, and this has beenachieved using layer-transfer techniques. A defect anneal step at about800° C. could be applied to repair defects caused to the layertransferred p− layer 207 from the ion implant step used for the layertransfer. The thickness of transferred p− layer 207 could be set to bevery thin, such as, for example, 5 nm or less and could be few tens ofnm such as 50 nm or 100 nm or even thicker. In many case the preferencecould be for thin or very thin to reduce the aspect ratio of thefollowing deep etch steps. These will be true for the many architecturesand flow/structure variations presented hereinafter.

Step (D): FIG. 2D illustrates the structure after Step (D). Usinglithography and then ion implantation, n+ regions 216 and p− regions 218may be formed on the transferred p− layer 207 after Step (C). Thislithography step and all the following lithography steps may be alignedto the underlying peripheral circuits 202. Alignment marks included inthose layer or layers could be used as the overlying layers oxide layer204, oxide layer 208 and transferred p− layer 207 are thin enough to sothe stepper could align to the alignment marks included in theperipheral circuits layers 202. The alignment accuracy could be betterthan 10 nm and far less than 40 nm. The oxide layer 204 and oxide layer208 thicknesses could be less than 100 nm and the transferred p− siliconlayer 207 could be less than 100 nm as well.

Step (E): FIG. 2E illustrates the structure after Step (E). An oxidelayer 220 may be deposited atop the structure obtained after Step (D). Afirst layer of Si/SiO₂ 222 is therefore formed atop the peripheralcircuit layer 202. The composition of the ‘SiO₂’ layer within thestacked Si/SiO₂ layers such as Si/SiO₂ 222, may be insulators ordielectrics other than silicon dioxide such as, for example, a low-kdielectric, carbon containing silicon oxides, amorphous carbon. Thethickness of the ‘SiO₂’ insulator layer within the stacked Si/SiO₂layers such as Si/SiO₂ 222 may be adjusted to minimize layer to layer,strata to strata disturb mechanisms, and may include thicknesses of 50nm, 75 nm, 100 nm, 150 nm, 200 nm, 500 nm, and less than 1000 nm. A thinconductive layer, such as a metal, may also be formed between the twobonding oxides to form a field shield to mitigate layer to layer, stratato strata disturb mechanisms, and may be electrically floating or tiedto a bias, such as ground or Vdd.

Step (F): FIG. 2F illustrates the structure after Step (F). Usingprocedures similar to Steps (B)-(E), additional Si/SiO₂ layers 224 and226 are formed atop Si/SiO₂ layer 222. A rapid thermal anneal (RTA) orspike anneal or flash anneal or laser anneal may then be done toactivate all implanted layers 222, 224 and 226 (and possibly also theperipheral circuit layer 202). Alternatively, the layers 222, 224 and226 may be annealed layer-by-layer as soon as their implantations aredone using a laser anneal system. The stacked Si/SiO₂ layers, such asSi/SiO₂ 222, may alternatively be formed by successive ion implants ofoxygen atoms/ions to various depths from the top surface of amono-crystalline silicon wafer/substrate and then heat treated to formoxide layers thus forming silicon layers in-between the oxide layers, alayered ‘SIMOX’ process approach.

Step (G): FIG. 2G illustrates the structure after Step (G). Lithographyand etch processes may then be utilized to make a structure such as, forexample, as shown in the FIG. 2G. The structure may include p− regions219 and N+ regions 217, any may be separated in the vertical directionfrom other conductive regions by a silicon oxide.

Step (H): FIG. 2H illustrates the structure after Step (H). Gatedielectric 228 and gate electrode 230 may then be deposited followingwhich a CMP may be done to planarize the gate electrode 230 regions.Lithography and etch may be utilized to define gate regions over the p−silicon regions (eg. p− Si region after Step (D)). Note that gate widthcould be slightly larger than p− region width to compensate for overlayerrors in lithography. Gate dielectric 228 and gate electrode 230 maybe, for example, a HKMG structure or a TEL SPA radical oxidation oxideand an appropriate work function electrode, for example, tungsten,degenerately doped polysilicon or amorphous silicon. Although the widthof the N+ regions 217 near and under the gates are illustrated as beinglarger than the thickness, one skilled in the art would recognize thatthe reverse (i.e., thickness larger than width) could be formed toprovide increased gate control of the transistor channel.

Step (I): FIG. 2I illustrates the structure after Step (I). A siliconoxide layer 232 may then be deposited and planarized. For clarity, thesilicon oxide layer is shown transparent in the figure, along withword-line (WL) and source-line (SL) regions.

Step (J): FIG. 2J illustrates the structure after Step (J). Bit-line(BL) contacts 234 are formed by etching and deposition. These BLcontacts may be shared among all layers of the memory. Bit line contactsmay be formed, for example, as sidewall structures or end-wallstructures, with various overlaps as required by the process and layout.

Step (K): FIG. 2K illustrates the structure after Step (K). Bit Linessuch as BL 236 may then be constructed. Contacts can made to BLs, WLsand SLs of the memory array at its edges. SL contacts can be made intostair-like structures using techniques described in “Bit Cost ScalableTechnology with Punch and Plug Process for Ultra High Density FlashMemory,” VLSI Technology, 2007 IEEE Symposium on, vol., no., pp. 14-15,12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; etal., following which contacts can be constructed to them. Formation ofstair-like structures for SLs could be done in steps prior to Step (K)as well.

FIG. 2L shows cross-sectional views of the array for clarity. Thedouble-gated transistors in FIG. 2 L can be utilized along with thefloating body effect for storing information.

FIG. 2M shows an illustration of a memory cell of the floating body RAMarray with two gates, such as gate electrode 230 and gate dielectric228, on either side of the p− region 219. Insulator region SiO₂ 238 mayprovide strata to strata isolation.

A floating-body DRAM has thus been constructed, with (1)horizontally-oriented transistors—i.e., current flowing in substantiallythe horizontal direction in transistor channels, (2) some of the memorycell control lines, e.g., source-lines SL, constructed of heavily dopedsilicon and embedded in the memory cell layer, (3) side gatessimultaneously deposited over multiple memory layers, and (4)monocrystalline (or single-crystal) silicon layers obtained by layertransfer techniques such as ion-cut.

FIGS. 3A-K describe another process flow to construct ahorizontally-oriented monolithic 3D DRAM. This monolithic 3D DRAMutilizes the floating body effect and double-gate transistors. No maskis utilized on a “per-memory-layer” basis for the monolithic 3D DRAMconcept shown in FIGS. 3A-K, and all other masks are shared betweendifferent layers. The process flow may include several steps in thefollowing sequence.

Step (A): Peripheral circuits with tungsten, or conventionalaluminum/copper, wiring 302 are first constructed and above this a layerof silicon dioxide 304 may be deposited. FIG. 3A shows a drawingillustration after Step (A).

Step (B): FIG. 3B illustrates the structure after Step (B). A wafer ofp− Silicon 308 has an oxide layer 306 grown or deposited above it.Following this, hydrogen may be implanted into the p− Silicon wafer at acertain depth indicated by 314. Alternatively, some other atomic speciessuch as Helium could be (co-)implanted. This hydrogen implanted p−Silicon wafer 308 may form top layer 310. The bottom layer 312 mayinclude the peripheral circuits 302 with oxide layer 304. The top layer310 may be flipped and bonded to the bottom layer 312 usingoxide-to-oxide bonding.

Step (C): FIG. 3C illustrates the structure after Step (C). The bondedstack of top and bottom wafers after Step (B) may be cleaved at thehydrogen plane 314 using either a anneal or a sideways mechanical forceor other means. A CMP process may then be conducted. A layer of siliconoxide 318 may be deposited atop the p− Silicon layer 316. Thus, asingle-crystal p− Si layer 316 may be atop the peripheral circuits, andthis has been achieved using layer-transfer techniques.

Step (D): FIG. 3D illustrates the structure after Step (D). Usingmethods similar to Step (B) and (C), multiple p− silicon layers 320 maybe formed with silicon oxide layers in between. The composition of the‘SiO₂’ layer within the stacked p-Si/SiO₂ layers may be insulators ordielectrics other than silicon dioxide such as, for example, a low-kdielectric, carbon containing silicon oxides, amorphous carbon. Thethickness of the ‘SiO₂’ insulator layer within the stacked p-Si/SiO₂layers may be adjusted to minimize layer to layer, strata to stratadisturb mechanisms, and may include thicknesses of 50 nm, 75 nm, 100 nm,150 nm, 200 nm, 500 nm, and less than 1000 nm. A thin conductive layer,such as a metal, may also be formed between the two bonding oxides toform a field shield to mitigate layer to layer, strata to strata disturbmechanisms, and may be electrically floating or tied to a bias, such asground or Vdd. The stacked Si/SiO₂ layers, may alternatively be formedby successive ion implants of oxygen atoms/ions to various depths fromthe top surface of a mono-crystalline silicon wafer/substrate and thenheat treated to form oxide layers thus forming silicon layers in-betweenthe oxide layers, a layered ‘SIMOX’ process approach.

Step (E): FIG. 3E illustrates the structure after Step (E) includingsilicon oxide regions 322. Lithography and etch processes may beutilized to make a structure, such as, for example, as shown in FIG. 3E.

Step (F): FIG. 3F illustrates the structure after Step (F). Gatedielectric 326 and gate electrode 324 may be deposited following which aCMP may be done to planarize the gate electrode 324 regions. Lithographyand etch are utilized to define gate regions. Gate dielectric 326 andgate electrode 324 may be, for example, a HKMG structure or a TEL SPAradical oxidation oxide and an appropriate work function electrode, forexample, tungsten, degenerately doped polysilicon or amorphous silicon.Although the width of the p− regions 321 near and under the gates areillustrated as being larger than the thickness, one skilled in the artwould recognize that the reverse (i.e., thickness larger than width)could be formed to provide increased gate control of the transistorchannel.

Step (G): FIG. 3G illustrates the structure after Step (G) including N+silicon regions 328. Using the hard mask (and remaining photoresist maybe utilized as part of the ‘hard mask’) defined in Step (F), p− regionsnot covered by the gate may be implanted to form n+ regions. Spacers maybe utilized during this multi-step implantation process and layers ofsilicon present in different layers of the stack may have differentspacer widths to account for lateral straggle of buried layer implants.Bottom layers could have larger spacer widths than top layers. A thermalannealing step, such as an RTA or spike anneal or laser anneal or flashanneal, may be conducted to activate the n+ implanted doped regions. Theoptical anneal system, such as the laser, spike, flash anneals, may beutilized to crystalize the polysilicon or amorphous silicon.

Step (H): FIG. 3H illustrates the structure after Step (H). A siliconoxide layer 330 may be deposited and planarized. For clarity, thesilicon oxide layer is shown transparent, along with word-line (WL) 332and source-line (SL) 334 regions.

Step (I): FIG. 3I illustrates the structure after Step (I). Bit-line(BL) contacts 336 are formed by etching and deposition. These BLcontacts may be shared among the layers of memory. Bit line contacts maybe formed, for example, as sidewall structures or end-wall structures,with various overlaps as required by the process and layout.

Step (J): FIG. 3J illustrates the structure after Step (J). BLs 338 maybe constructed. Contacts may be made to BLs, WLs and SLs of the memoryarray at its edges. SL contacts can be made into stair-like structuresusing techniques described in “Bit Cost Scalable Technology with Punchand Plug Process for Ultra High Density Flash Memory,” VLSI Technology,2007 IEEE Symposium on, vol., no., pp. 14-15, 12-14 Jun. 2007 by Tanaka,H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., following which contactscan be constructed to them. Formation of stair-like structures for SLscould be done in steps prior to Step (J) as well.

FIG. 3K shows cross-sectional views of the array for clarity.Double-gated transistors may be utilized along with the floating bodyeffect for storing information.

A floating-body DRAM has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in transistor channels (2) some of the memorycell control lines, e.g., source-lines SL, constructed of heavily dopedsilicon and embedded in the memory cell layer, (3) side gatessimultaneously deposited over multiple memory layers, and (4)monocrystalline (or single-crystal) silicon layers obtained by layertransfer techniques such as ion-cut.

With the explanations for the formation of monolithic 3D DRAM withion-cut in this section, it is clear to one skilled in the art thatalternative implementations are possible. BL and SL nomenclature hasbeen used for two terminals of the 3D DRAM array, and this nomenclaturecan be interchanged. Each gate of the double gate 3D DRAM can beindependently controlled for better control of the memory cell. Toimplement these changes, the process steps in FIGS. 2 and 3 may bemodified. Moreover, selective epi technology or laser recrystallizationtechnology could be utilized for implementing structures shown in FIG.2A-M and FIG. 3A-K. Various other types of layer transfer schemes andanneal/shielding techniques that have been described in the referencedand incorporated patents can be utilized for construction of various 3DDRAM structures. Furthermore, buried wiring, i.e. where wiring formemory arrays is below the memory layers but above the periphery, mayalso be used. In addition, other variations of the monolithic 3D DRAMconcepts are possible, such as, for example, forming the peripherycircuits above and/or below the memory stack layers.

The positive charge stored, such as in the floating body DRAM describedin FIGS. 2-3, may decrease over time due to the diode leakage current ofthe p-n junctions formed between the floating body and n+ regions anddue to charge recombination. A method to refresh all memory cells inparallel have been described in “Autonomous Refresh of Floating BodyCell (FBC)”, Ohsawa et al., pp. 801-804, International Electron DeviceMeeting, 2008 (“Ohsawa”), U.S. Pat. No. 7,170,807 “Data Storage Deviceand Refreshing Method for Use with Such Device”, Fazan et al. (“Fazan”)and in U.S. Pat. No. 8,264,875 “A Semiconductor Memory Device Having anElectrically Floating Body Transistor”, Widjaja and Or-Bach (“Widjaja”),which are incorporated by reference herein in entirety. Ohsawa and Fazanteach an autonomous refresh method by applying a periodic gate and drainvoltage pulses and Widjaja describes a parallel refresh method byapplying a bias to the back bias terminal.

FIG. 4A is a drawing illustration of the cross-section of a memory cell450 in a memory layer of the 3D DRAM device shown in FIG. 2L. FIG. 4B isa drawing illustration of the equivalent circuit representation of thememory cell shown in FIG. 4A. The refresh operation will be describedwith reference to FIGS. 4A and 4B, although the same principle appliesto other 3D DRAM described herein, for example, as shown in FIG. 3K.

As shown in FIG. 4A, memory cell 450 may include a floating body 424 ofp-type conductivity. The floating body 424 may be bounded by aninsulating region 422, n+ regions 416 and 418, and by the surface 414. Agate 460 may be positioned in-between the n+ regions 416 and 418, andmay be insulated from the floating body region 424 by an insulatinglayer 462, and may be connected to gate connectivity 470.

As shown in FIG. 4B, inherent in the each of the floating body DRAMmemory cells 450 is n-p-n bipolar device 430 formed by n+ region 416(the collector region), floating body 424 (the base region), and n+region 418 (the emitter region). The collector n+ region 416 can beconnected to the SL terminal 472 and the emitter region 418 can beconnected to the BL terminal 474, or vice versa. The refresh operationcan be performed by applying a positive voltage to the collector n+region 416 while simultaneously grounding the emitter region 418. Therefresh operation is relatively independent of the voltage applied togate 460. In some embodiments of the invention, the gate may begrounded.

If floating body 424 is sufficiently positively charged, a statecorresponding to logic-1, the bipolar device 430 will be turned on. Inparticular, the voltage across the reversed biased p-n junction betweenthe floating body 424 and the collector n+ region 416 may cause a smallcurrent to flow across the junction. Some of the current may be in theform of hot carriers accelerated by the electric field across thejunction. These hot carriers may collide with atoms in the semiconductorlattice which may generate hole-electron pairs in the vicinity of thejunction. The electrons may be swept into the collector n+ region 416 bythe electric field, while the holes may be swept into the floating bodyregion 424.

The hole current flowing into the floating region 424 (usually referredto as the base current) will maintain the logic-1 state data. Theefficiency of the refresh operation can be enhanced by designing thebipolar device formed by collector n+ region 416, floating region 424,and emitter region 418 to be a low-gain bipolar device, where thebipolar gain is defined as the ratio of the collector current flowingout of collector n+ region 416 to the base current flowing into thefloating region 424.

FIG. 4C is a drawing illustration of the energy band diagram of theintrinsic n-p-n bipolar device 430 when the floating body region 424 ispositively charged and a positive bias voltage is applied to thecollector n+ region 416. The dashed lines indicate the Fermi levels inthe various regions of the n-p-n transistor 430. The Fermi level islocated in the band gap between the solid line 417 indicating the top ofthe valance band (the bottom of the band gap) and the solid line 419indicating the bottom of the conduction band (the top of the band gap)as is well known in the art. The positive charge in the floating bodyregion lowers the energy barrier of electron flow into the base region.Once injected into the floating body region 424, the electrons will beswept into the collector n+ region 416 due to the positive bias appliedto the collector n+ region 416. As a result of the positive bias, theelectrons may be accelerated and create additional hot carriers (hothole and hot electron pairs) through an impact ionization mechanism. Theresulting hot electrons flow into the collector n+ region 416 while theresulting hot holes will subsequently flow into the floating body region424. This process restores the charge on floating body 424 and willmaintain the charge stored in the floating body region 424 which willkeep the n-p-n bipolar transistor 430 on for as long as a positive biasis applied to the collector n+ region 416.

If floating body 424 is neutrally charged (the voltage on floating body424 being equal to the voltage on grounded emitter region 418), a statecorresponding to logic-0, no appreciable current will flow through then-p-n transistor 430. The bipolar device 430 will remain off and noappreciable impact ionization occurs. Consequently memory cells in thelogic-0 state will remain in the logic-0 state.

FIG. 4D shows the energy band diagram of the intrinsic n-p-n bipolardevice 430 when the floating body region 424 is neutrally charged and abias voltage is applied to the collector n+ region 416. In this statethe energy level of the band gap bounded by solid lines 417A and 419A isdifferent in the various regions of n-p-n bipolar device 430. Becausethe potential of the floating body region 424 and the emitter region 418is substantially equal, the Fermi levels are constant, resulting in anenergy barrier between the emitter region 418 and the floating bodyregion 424. Solid line 423 indicates, for reference purposes, the energybarrier between the emitter region 418 and the floating body region 424.The energy barrier prevents electron flow from the emitter region 418 tothe floating body region 424. Thus the n-p-n bipolar device 430 willremain off.

FIG. 4E is a drawing illustration of an exemplary refresh operation inan exemplary array 480 formed by a plurality of memory cell 450(corresponds to a memory layer of the 3D DRAM device, such as thosedescribed herein) is shown in FIG. 4E. Present in FIG. 4E are word lines(WLs) 470 a through 470 n, source lines (SLs) 472 a through 472 n, andbit lines (BLs) 474 a through 474 p. In this example, each of the wordlines 470 a through 470 n is associated with a single row of memorycells 450 and is coupled to the gate 460 of each memory cell 450 in thatrow. Similarly, each of the source lines 472 a through 472 n isassociated with a single row of memory cells 450 and is coupled to thecollector n+ region 416 of each memory cell 450 in that row. Each of thebit lines 474 a through 474 p is associated with a single column ofmemory cells 450 and is coupled to the emitter region 418 of each memorycell 450 in that column. In this exemplary refresh operation, there isno individually selected memory cell. Rather cells are selected in rowsby the source lines 472 a through 472 n and may be selected asindividual rows, as multiple rows, or as all of the rows comprisingarray 480. Cells can also be selected in columns by bit lines 474 athrough 474 p and may be selected as individual columns, as multiplecolumns, or as all of the columns comprising array 480.

Persons of ordinary skill in the art will appreciate that whileexemplary array 480 is shown as a single continuous array in FIG. 4E,that many other organizations and layouts are possible like, forexample, word lines may be segmented or buffered, bit lines may besegmented or buffered, source lines may be segmented or buffered, thearray 480 may be broken into two or more sub-arrays, control circuitssuch as word decoders, column decoders, segmentation devices, senseamplifiers, write amplifiers may be arrayed around exemplary array 480or inserted between sub-arrays of array 480. Thus the exemplaryembodiments, features, design options, etc., described are not limitingin any way.

Also shown in FIG. 4E are multiplexers 440 a through 440 n and voltagewaveforms 442 a through 442 n. A constant positive bias can be appliedto the collector n+ regions 416 of memory cells 450 (through SL terminal472). Alternatively, a periodic pulse of positive voltage can be appliedto the collector n+ regions 416 of memory cells 450 through SL terminal472 as opposed to applying a constant positive bias to reduce the powerconsumption of the memory cell 450. FIG. 4E further shows multiplexers440 a through 440 n each coupled to one of the source lines 472 athrough 472 n that determine the bias voltages applied to SL terminals472 a through 472 n, which will be determined by different operatingmodes. The pulsing of the voltage on the SL terminals may be controlled,for example, by applying pulses of logic signals such as, for example,waveforms 442 a through 442 n to the select input of multiplexers 440 athrough 440 n thereby selecting, for example, ground (0.0 volts) or apower supply voltage such as V_(CC). Many other techniques may be usedto pulse the voltage applied to SL terminals 472 a through 472 n suchas, for example, applying the waveforms 442 a through 442 n at differenttimes, or applying them simultaneously, or coupling the select inputs ofmultiplexers 442 a through 442 n together and applying a single pulsedwaveform to all of the multiplexers 442 a through 442 n simultaneously(not shown in the figure). Many other options will readily suggestthemselves to persons of ordinary skill in the art. Thus the describedexemplary embodiments are not limiting in any way.

FIG. 4F is a drawing illustration of another method to provide voltagepulses to SL terminals 472 a through 472 n of exemplary array 480 ofmemory cells 450. The positive input signals to multiplexers 440 athrough 440 n may be generated by voltage generator circuits 444 athrough 444 n coupled to one input of each of the multiplexers 440 athrough 440 n. Alternatively, a single voltage generator circuit may becoupled to each of the multiplexers 440 a through 440 n reducing theamount of overhead circuitry required to refresh the memory cells 450 ofarray 480. Other embodiments are possible including, for example,applying the waveforms 442 a through 442 n at different times, orapplying them simultaneously, or coupling the select inputs ofmultiplexers 442 a through 442 n together and applying a single pulsedwaveform to all of the multiplexers 442 a through 442 n simultaneously(not shown in the figure).

FIG. 4G is a drawing illustration of a reference generator circuitsuitable for use as reference generator circuits 444 a through 444 n inFIG. 4F. The reference generator includes reference cell 453, which mayconsist of a modified version of memory cell 450 described above withregion 425 of p-type conductivity. The p-type region 425 allows for adirect sensing of the floating body region 424 potential. P-type region425 is drawn separately even though it has the same conductivity type asfloating body region 424 because it may be doped differently, such as,for example, a higher concentration of doping than floating body region424, to facilitate contacting it. The reference cell 453 for example canbe configured to be in state logic-1 where the potential of the floatingbody region 424 is positive, for example at +0.5V. The potential sensedthrough the p-type region is then compared with a reference valueV_(REF), e.g. +0.5V, by operational amplifier 427. If the potential ofthe floating body region 424 is less than the reference value, thevoltage applied to the SL terminal 472 (which is connected to drain n+region 416 of the reference cell 453 and is also connected to collectorn+ region 416 of the memory cell 450) may be increased by operationalamplifier 427 until the potential of the floating body region 424reaches the desired reference voltage. If the potential of the floatingbody 424 region is higher than that of the reference value, the voltageapplied to SL terminal 472 can be reduced by operational amplifier 427until the potential of the floating body region 424 reaches the desiredreference voltage. Reference voltage V_(REF) may be generated in manydifferent ways such as, for example, using a band gap reference, aresistor string, a digital-to-analog converter, and so on. Similarlyalternate voltage generators of types known in the art may be used.

Also shown in FIGS. 4E and 4F, read circuitry 499 a may be coupled tothe bit line 474 a. In some embodiments, a reading circuit 499 b through499 p (not shown in FIGS. 4E and 4F) may be coupled to each bit of thebit lines 474 b through 474 p, while in other embodiments readingcircuit 499 a may be shared between multiple columns using a decodingscheme (not shown).

FIG. 5A is a drawing illustration of another embodiment of a gatelessmemory cell 5150 in a memory layer of the 3D DRAM device. FIG. 5B is adrawing illustration of the equivalent circuit representation of thememory cell shown in FIG. 5A. Similar to memory cell 450 shown in FIG.4A, memory cell 5150 includes a floating body 524 of p-typeconductivity. The floating body 524 is bounded by the insulator layer522, n+ regions 516 and 518, and by the insulator layer 526. Adifference to the memory cell 450 shown in FIG. 4A is that no gateelectrode exists in memory cell 5150. The n+ regions 516 and 518 may beconfigured differently, for example, the n+ region acting as the emitterregion (n+ region 518) is typically more heavily doped than the n+collector region (n+ region 516).

As shown in FIG. 5B, floating body DRAM cell 5150 may include an n-p-nbipolar device 5130, n-p− n bipolar device 5130 may include n+ region516 (the collector region), floating body 524 (the base region), and n+region 518 (the emitter region). The collector region can be connectedto the SL terminal 572 and the emitter region can be connected to the BLterminal 574, or vice versa.

FIG. 5C is a drawing illustration of an exemplary memory array 5180which will be used in subsequent drawing figures to illustrate thevarious operations that may be performed on memory cell 5150 whenarranged in an array to create a memory device. Memory array 5180comprises in part representative memory cells 5150 a, 5150 b, 5150 c,and 5150 d. In operations where a single memory cell is selected,representative memory cell 5150 a will represent the selected cell whilethe representative memory cells 5150 b, 5150 c, and 5150 d willrepresent the various cases of unselected memory cells sharing a row,sharing a column, or sharing neither a row nor a column respectivelywith selected representative memory cell 5150 a. Similarly in the caseof operations performed on a single row or column, representative memorycell 5150 a will be on the selected row or column.

The memory cell states are represented by the charge in the floatingbody 524, which modulates the intrinsic n-p-n bipolar device 5130. Thecollector region can be connected to the SL terminal 572 and the emitterregion can be connected to the BL terminal 574, or vice versa. If cell5150 has a substantial number of holes stored in the body region 524,then the memory cell may have a higher bipolar current (e.g. currentflowing from BL to SL terminals during read operation) compared to ifcell 5150 does not store an appreciable amount of holes in body region524.

The positive charge stored in the body region 524 may decrease over timedue to the p-n diode leakage formed by floating body 524 and n+ regions516 and 518, and due to charge recombination. A refresh operationapplied to the entire memory array 5180 is illustrated in FIG. 5D. Therefresh operation on memory cell 5150 can be performed by applying apositive bias to the collector region 516 (connected for example to SLterminal 572) while grounding the emitter region 518 (connected forexample to BL terminal 574). If floating body 524 is substantiallypositively charged (i.e. in a state logic-1), the n-p-n bipolartransistor 5130 will be turned on.

A fraction of the bipolar transistor 5130 current will then flow intofloating body region 524 and maintain the state logic-1 data. Theefficiency of the refresh operation can be enhanced by designing thebipolar transistor 5130 to be a low-gain bipolar device, where thebipolar gain is defined as the ratio of the collector current flowingout of collector region 516 to the base current flowing into floatingbody region 524.

For memory cells in state logic-0 data, the bipolar device will not beturned on, and consequently no appreciable base hole current will flowinto floating region 524. Therefore, memory cells in state logic-0 willremain in state logic-0.

A periodic pulse of positive voltage can be applied to the SL terminal572 as opposed to applying a constant positive bias to reduce the powerconsumption of the memory cell 5150.

In the entire array refresh operation of FIG. 5D, source line terminals572 a through 572 n may be biased at +1.2V and bit lines 574 a through574 p may be biased to 0.0V. This refreshes substantially all of thecells in memory array 5180.

A single row operation can also be performed on memory array 5180 asillustrated in FIG. 5E, where selected source line terminal 572 a may bebiased at +1.2V while the unselected source line terminals 572 b (notshown) through 572 n may be biased at Vdd/2, and bit lines 574 a through574 p may be biased to 0.0V. This refreshes substantially all of theselected cells in memory array 5180.

A single memory cell read operation is illustrated in FIG. 5F. The readoperation for memory cell 5150 a, for example, can be performed bysensing the current of the bipolar device 5130 by applying a positivevoltage to the selected BL terminal 574 a and zero voltage to theselected SL terminal 572 a. The positive voltage applied to the selectedBL terminal may be less than or equal to the positive voltage applied tothe SL terminal during the refresh operation. The unselected BLterminals may remain at Vdd/2 and the unselected SL terminals may remainat a positive voltage.

FIG. 5F shows the bias condition for the selected memory cell 5150 a andunselected memory cells 5150 b, 5150 c, and 5150 d in memory array 5180.In this particular non-limiting embodiment, about 0.0 volts may beapplied to the selected SL terminal 572 a while about +1.2V is appliedto the unselected source line terminals 572 b (not shown) through 572 n,about +1.2 volts is applied to the selected BL terminal 574 a while 0.0Vis applied to the unselected bit line terminals 574 b through 574 p.These voltage levels are exemplary only and may vary from embodiment toembodiment.

Current will flow through intrinsic bipolar device 5130 if the floatingbody is substantially positively charged and no current to flow if thefloating body is substantially discharged since the bipolar device 5130is off. As described above, the n+ regions 516 and 518 can be configuredasymmetrically such that the current flowing through the selected cell5150 a during read operation (from the BL terminal 574 to the SLterminal 572) may be higher than the refresh current flowing through theunselected memory cells (from the SL terminal 572 to the BL terminal574), although similar bias conditions are applied to selected andunselected memory cells 5150 (with the bias conditions are reversedbetween the BL and SL terminals of the selected and unselected memorycells 5150).

For memory cell 5150 b sharing the same row as the selected memory cell5150 a, the SL terminal 572 a and the BL terminal 574 p are both biasedto 0.0V and consequently these cells will not be at the refresh mode.However, because read operation is accomplished much faster (in theorder of nanoseconds) when compared to the lifetime of the charge in thefloating body 524 (in the order of milliseconds), it should cause littledisruption to the charge stored in the floating body.

For memory cell 5150 c sharing the same column as the selected memorycell 5150 a, a positive voltage is applied to the BL terminal 574 a andSL terminal 572 n. No base current will flow into the floating body 524because there is no appreciable potential difference between SL terminal572 and BL terminal 574 (i.e. the emitter and collector terminals of then-p-n bipolar device 5130). However, because the read operation isaccomplished much faster (in the order of nanoseconds) compared to thelifetime of the charge in the floating body 524 (in the order ofmilliseconds), it should cause little disruption to the charge stored inthe floating body.

For memory cell 5150 d sharing neither the same row nor the same columnas the selected memory cell 5150 a, both the SL terminal 572 n willremain positively charged and the BL terminal 574 p may remain grounded.Representative memory cell 5150 d will be in the refresh mode, wherememory cells in state logic-1 will maintain the charge in floating body524 because the intrinsic bipolar device 5130 will generate hole currentto replenish the charge in floating body 524, while memory cells instate logic-0 will remain in neutral state.

The various voltage bias levels above are exemplary only. They will varyfrom embodiment to embodiment as a function of both design choice andthe process technology used.

FIG. 5G illustrates a single row write logic-0 operation. In FIG. 5G theselected row SL terminal 572 a may be biased negatively at about −0.5Vwhile the unselected row SL terminals 572 b (not shown) through 572 nmay be biased at about +1.2V and all the BL terminals 574 a through 574p may be biased at 0.0V. This causes the selected cells such asrepresentative memory cells 5150 a and 5150 b to have their bipolardevices turn on due to forward bias on the floating body 524 tocollector region 516, thereby evacuating the holes from the floatingbody 524.

For the unselected rows (which in this case is all the memory cells 5150in memory array 5180 not on the selected row), the SL terminal 572 is at+1.2V and the BL terminal 574 is at 0.0V, which corresponds to therefresh operation described above.

A write logic-0 operation can also be performed on a column basis byapplying a negative bias to the BL terminal 574 as opposed to the SLterminal 572. The SL terminal 572 will be zero or positively biased.Under these conditions, substantially all memory cells sharing the sameBL terminal 574 will be written into state logic-0 and substantially allthe other cells will be in the refresh operation.

The various voltage bias levels above are exemplary only. They will varyfrom embodiment to embodiment as a function of both design choice andthe process technology used.

A write logic-1 operation can be performed on memory cell 5150 throughan impact ionization mechanism as described in FIG. 5H. An example ofthe bias condition of the selected memory cell 5150 a under impactionization write logic-1 operation is illustrated in FIG. 5H. A positivebias may be applied to the BL terminal 574, while zero voltage may beapplied to the selected SL terminal 572. The positive bias applied tothe BL terminal 574 may be greater than the positive voltage applied tothe SL terminal 572 during refresh operation. The positive bias appliedto the BL terminal is large enough to turn on bipolar device 5130regardless of the initial state of the data in selected memory cell 5150a. This results in a base hole current to the floating body 524 of theselected memory cell 5150 a charging it up to a logic-1 state.

In one particular non-limiting embodiment, the following bias conditionsmay be applied to the selected memory cell 5150 a: a potential of about0.0 volts is applied to selected SL terminal 572 a and a potential ofabout +2.0 volts is applied to selected BL terminal 574 a. The followingbias conditions may be applied to the unselected terminals: about +1.2volts is applied to SL terminals 572 b (not shown) through 572 n, andabout 0.0 volts is applied to BL terminals 574 b through 574 p. FIG. 5Hshows the bias condition for the selected and unselected memory cells inmemory array 5180. The various voltage bias levels above are exemplaryonly. They will vary from embodiment to embodiment as a function of bothdesign choice and the process technology used.

For representative memory cell 5150 b sharing the same row as theselected memory cell 5150 a, SL terminal 572 a and BL terminal 574 p maybe grounded. Bipolar device 5130 will be off and the memory cell 5150 bwill not be at the refresh mode. However, because write operation isaccomplished much faster (in the order of nanoseconds) compared to thelifetime of the charge in the floating body 524 (in the order ofmilliseconds), it should cause little disruption to the charge stored inthe floating body.

For representative memory cell 5150 c sharing the same column as theselected memory cell 5150 a, a greater positive voltage is applied tothe BL terminal 574 a and a lesser positive voltage is applied to SLterminal 572 n. Less base current will flow into the floating body 524than in selected memory cell 5150 a because of the lower potentialdifference between SL terminal 572 and BL terminal 574 (i.e. the emitterand collector terminals of the n-p-n bipolar device 5130). However,because the write operation is accomplished much faster (in the order ofnanoseconds) compared to the lifetime of the charge in the floating body524 (in the order of milliseconds), it should cause little disruption tothe charge stored in the floating body.

For representative memory cell 5150 d sharing neither the same columnnor the same row as the selected memory cell 5150 a, the SL terminal 572is positively charged and the BL terminal is grounded. Representativememory cell 5150 d will be at refresh mode, where memory cells in statelogic-1 will maintain the charge in floating body 524 because theintrinsic bipolar device 5130 will generate hole current to replenishthe charge in floating body 524 and where memory cells in state logic-0will remain in neutral state.

The various voltage bias levels above are exemplary only. They will varyfrom embodiment to embodiment as a function of both design choice andthe process technology used. Also, the first conductivity type may bechanged from p-type to n-type and the second conductivity type may bechanged from n-type to p-type, and the polarities of the applied biasesmay be reversed. Thus the invention is not to be limited in any wayexcept by the appended claims.

An important feature of a DRAM device is a low refresh rate. It is evendesirable if a two stable states memory could be achieved withoutinterfering with the memory access for read and write, as it wouldremove the necessity of a refresh operation. Such a memory could becalled SRAM (Static Random Access Memory). There are some 2D structuresthat presented such type of memories; for example, K. Sakui, T.Hasegawa, T. Fuse, S. Watanabe, K. Ohuchi, and F. Masuoka, “A new staticmemory cell based on the reverse base current effect of bipolartransistors,” IEEE Trans. Electron Devices, vol. 36, no. 6, pp.1215-1217, June 1989; M. Reisch, “On bistable behavior and open-basebreakdown of bipolar transistors in the avalanche regime-Modeling andapplications,” IEEE Trans. Electron Devices, vol. 39, no. 6, pp.1398-1409, June 1992; and US Patent Publication No. 2012/0230123 “Methodof Maintaining the State of Semiconductor Memory Having ElectricallyFloating Body Transistor”. These three documents are incorporated byreference herein in their entirety.

FIG. 6 is a drawing illustration of a Floating Body memory cell 50 witha buried well region 12, where applying a back bias (through terminal78) to the n-type buried well region 12 (also referred to as the backbias region 12) may maintain the charge stored in the floating bodyregion 24. FIG. 6 may be found as FIG. 1A of the US Patent PublicationNo. 2012/0230123. The floating body 24 may be P type, the drain 16 andsource 18 may be N type, the gate oxide 62 and the gate 60 may include,such as, for example, SiO₂ and polysilicon or HKMG, and the floatingbody transistor may be isolated by deep trench isolations 26. The userterminals for writing and reading the memory may be drain 72, source 74,and gate 70. The top surface 14 of the mono-crystal may be the upperside of the wafer. Interconnect layers, such as is known by thoseskilled in the art, may be formed to connect the memory cell (notshown). A key to the two charge states holding of the floating bodymemory is the application of a positive back bias, such as 1.2 volt, tothe back bias region 12 (through terminal 78). The NPN bipolartransistor formed between the n-type back bias region 12, the p-typefloating body region 24, and the n-type drain 16 or source 18 is the keyto the unique charging mechanism as described in these referenced artand before in respect to at least FIGS. 4A, 4B, 5A and 5B. A key to theself-charging mechanism, as had been detailed in the referenced art, ishaving the structure meet the formula (M−1)*3−1, which is related to thestructure feature and design of the memory cell 50, where β is the gainof the bipolar transistor and M is the impact ionization multiplicationfactor.

In the following we present a 3D RAM architecture having two stablestates, the two stable states may be two stable floating body chargelevels. The structure and process flow have similarities to a previousdescription, such as FIGS. 3A-K and associated text. A device and methodto form back-bias region for 3D floating body memory architecture isdescribed below.

As illustrated in FIGS. 7A, 7B, 7C, and 7D, FIG. 7A is a drawingillustration of an embodiment of a three-dimensional view of a 3D RAMstructure and device architecture prior to the interconnect stage of theprocess flow (similar to FIG. 7G), FIG. 7B is a drawing illustration ofcross-sectional cut I of FIG. 7A, FIG. 7C is a drawing illustration ofcross-sectional cut II of FIG. 7A, and FIG. 7D is a drawing illustrationof cross-sectional cut II of FIG. 7A with the gate material andperforated gate dielectric overlap/under-lap depicted. FIGS. 7A, 7B, 7C,and 7D may include n+ silicon regions 728, silicon oxide regions 722,gate regions 760, back-bias regions 712, gate dielectric regions 762, p−channel regions 724, isolation dielectric 706, gate dielectric openings711, and peripheral circuits 702. FIG. 7D may include a shadowed overlayimage of gate regions 760 and gate dielectric openings 711 depicting theoverlap and underlaps of these regions with respect to p− channelregions 724. FIG. 7B may include surface 713, wherein back-bias region712 may be in physical contact and make electrical connection to p−channel region 724.

A floating body transistor in the stack may include a single side gate(gate regions 760) on one side of floating body p− channel region 724and a back-bias region 712 formed on the other side of the floating bodyp− channel region 724. Gate region 760 may be insulated from thefloating body p− channel region 724 with a gate dielectric regions 762,yet still modulate the channel. In this case the back-bias is nothorizontal such as in FIG. 4A but rather vertical such as back biasregions 712 of FIG. 7B. N+ silicon regions 728 may be connected andutilized as either a source or drain to help form the two stable statesfloating body transistor.

A process flow that may be utilized to form the structure shown in FIGS.7A-7D is described as follows. Many steps may be similar to those usedto form the structure shown in FIG. 3E.

As illustrated in FIG. 7E, multiple mono-crystalline p− silicon layersmay be formed with silicon oxide layers in-between and then lithographyand etch processes may be utilized to make a structure, such as, forexample, as shown in FIG. 7E. Thus p− silicon regions 721 and siliconoxide regions 722 may be stacked and formed above peripheral circuits702. Processes to form the structure in FIG. 7E may include the stepsdescribed with respect to FIGS. 3A to 3E herein. Peripheral circuits 702may be constructed with tungsten, or conventional aluminum/copper,wiring and may have isolation and/or bonding oxide above it (in-betweenthe top metallization/wiring of peripheral circuits 702 and the bottomp-silicon region in the stack, such as isolation oxide 706. Thecomposition of the silicon oxide regions 722 within the stacked Si/SiO₂layers may be insulators or dielectrics other than silicon dioxide suchas, for example, a low-k dielectric, carbon containing silicon oxides,amorphous carbon. The thickness of silicon oxide regions 722 within thestacked Si/SiO₂ layers may be adjusted to minimize layer to layer,strata to strata disturb mechanisms, and may include thicknesses of 50nm, 75 nm, 100 nm, 150 nm, 200 nm, 500 nm, and less than 1000 nm. A thinconductive layer, such as a metal, may also be formed between the twobonding oxides to form a field shield to mitigate layer to layer, stratato strata disturb mechanisms, and may be electrically floating or tiedto a bias, such as ground or Vdd. The stacked Si/SiO₂ layers mayalternatively be formed by successive ion implants of oxygen atoms/ionsto various depths from the top surface of a mono-crystalline siliconwafer/substrate and then heat treated to form oxide layers thus formingsilicon layers in-between the oxide layers, a layered ‘SIMOX’ processapproach.

As illustrated in FIG. 7F, a perforated gate dielectric layer 762(partially shown for clarity) may be formed which has gate dielectricopenings 711 on one face of the p− silicon 721 and silicon oxide 722layer stack, a gate electrode material may be deposited and CMP'dsubstantially to the top of the Si/SiO₂ stack. The gate dielectric maybe grown on the exposed surfaces of p− silicon regions 721 (for example,with the TEL SPA tool radical oxidation oxide) or depositedsubstantially over the structure's surface, such as, for example, in thewell-known HKMG formation processing, a lithographic step and etch ofthe deposited gate dielectric may thus form perforated gate dielectriclayer 762 and gate dielectric openings 711 on one face of the p− silicon721 and silicon oxide 722 layer stack and leave the gate dielectricintact another face of the p− silicon 721 and silicon oxide 722 layerstack. A gate electrode material, such as, for example, doped amorphousor polysilicon, or the well-known metal stack of HKMG processing, may bedeposited, thus bringing the conductive gate electrode material indirect contact and electrical connection with exposed faces of thep-silicon 721, and forming a field effect device of gate electrodeinfluence thru the gate dielectric of the eventual transistor p− channelregions 724 formed by portions of p-silicon 721 layers. The gateelectrode material may be in-situ doped during deposition, or doped at alater step. CMP may be performed to planarize the gate electrodematerial. The top face of the topmost silicon oxide region 722 may havean etch stop/CMP stop material, such as, for example, SiN or a-carbon,placed on it at an earlier stage, so to provide a CMP stop, thusenabling the formation of separate gate and back bias control regions.Although the width of the p− regions 721 near and under the gates areillustrated as being larger than the thickness, one skilled in the artwould recognize that the reverse (i.e., thickness larger than width)could be formed to provide increased gate control of the transistorchannel.

As illustrated in FIG. 7G, n+ silicon regions 728, gate regions 760 andback-bias regions 712 may be formed. The gate may be lithographydefined. The widths of the gate structure regions and the gatedielectric openings 711 may be designed such that the gate structureregions will substantially always overlap the gate dielectric openings711. Using the remaining photoresist of the gate structure regionslithography, portions of p− regions 721 not covered by the gatestructure regions photoresist may be implanted to form n+ regions 728,and thus form p− channel regions 724. This multi-step implantationprocess may utilize different implant energies. The gate may then beetched to define gate structure regions shown in FIG. 7G, thus forminggate regions 760 and back-bias regions 712, and the photoresiststripped. A thermal annealing step, such as an RTA or spike anneal orlaser anneal or flash anneal, may be conducted to activate the n+implanted doped regions. The optical anneal system, such as the laser,spike, flash anneals, may be utilized to crystalize any depositedpolysilicon or amorphous silicon (which may be used as a gate electrodematerial depending on process design choice). The structure valleys maybe filled with a dielectric (not shown for clartity), such as, forexample, SACVD oxides, and then CMP'd substantially to or partially intothe topmost silicon oxide layer 722 of the Si/SiO2 stack, orsubstantially to or partially into the CMP/etch stop layer as previouslydiscussed. Thus, horizontal floating gate transistors with separate gateand back bias control regions on the previously exposed sides of each p−transistor channel region, and horizontal select lines (as has beendescribed before in connection to FIG. 3 herein, and in the incorporatedpatent references) may be formed in a stack configuration.

As illustrated in FIG. 7H, an inter-layer dielectric, such as, forexample silicon oxide or doped silicon dioxides, may be deposited andplanarized. For clarity, the silicon oxide layer is shown transparent.Contacts to gate regions 760 and the associated local metallization gateconnectivity 750, as well as contacts to back-bias regions 712 and theassociated local metallization back-bias connectivity 752 may be formedusing known methods in the art.

As illustrated in FIG. 7I, Source-Line (SL) connectivity 754, such ascontacts and metal lines, may be formed by etching and deposition usingknown methods in the art. These SL contacts may be shared among thelayers of memory. Gate connectivity 750 and back-bias connectivity 752are not shown in FIG. 7I for clarity. The bit lines (BL) 756 are shownas a portion of the n+ silicon regions 728. Bit line contacts may beformed, for example, as sidewall structures or end-wall structures, withvarious overlaps as required by the process and layout.

Further, using known methods in the art, gate connectivity 750 may beconnected to form the WL—word select lines. Contacts may be made to BLs,WLs and SLs of the memory array at its edges. BL contacts can be madeinto stair-like structures using techniques described in “Bit CostScalable Technology with Punch and Plug Process for Ultra High DensityFlash Memory,” VLSI Technology, 2007 IEEE Symposium on, vol., no., pp.14-15, 12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.;et al., following which contacts can be constructed to them. Formationof stair-like structures for BLs could be done in steps prior to FIG.7I. Electrical connection to the underlying peripheral circuits may beaccomplished with vertical conductive vias, for example, the thru layervia or thru silicon via processes and structures, which may be describedin the incorporated patent references or known by those skilled in theart. In addition, thermal vias may be utilized to carry heat, whetherfrom processing or from operation, away from the upper layer withoutharming the underlying metallization or devices.

FIG. 7J is a top view exemplary illustration of the floating bodytransistors sharing on one side of the Si/SiO₂ stacks (n+ regions 728and floating body p− channel regions 724) gate regions 760 and on theother side of the Si/SiO₂ stacks back bias regions 712 connected to p−channel regions 724 in the openings of the perforated gate dielectriclayer 762. The back-bias regions act as the collector of the bipolarcharging transistor as illustrated in FIG. 4B. Bipolar transistors thatutilize a polysilicon collector with monocrystalline base and emitterare well known in the art to perform well. The contacts of the sourceline connectivity 754 are shown, but the metal lines, as well as the SLstaircase and WL connectivity are not shown for clarity. Back-biasregions 712 may be mutually connected to a bias source.

Persons of ordinary skill in the art will appreciate that theillustrations in FIGS. 7A through 7J are exemplary only and are notdrawn to scale. Such skilled persons will further appreciate that manyvariations are possible such as, for example, the Si/SiO₂ stack may beformed with the N+ and p− regions already formed by stacking with 1-maskper stack layer processing as described, for example, in FIG. 2 herein.Further, it may not be necessary to overlap the implant gate structureregions and the gate dielectric openings 711. Moreover, perforated gatedielectric layer 762 may not need to be perforated, and control of theback-bias effects may be performed thru the gate dielectric from anefield influence modulated by the back bias regions 712, or by bandbending with an Esaki diode structure. Furthermore, peripheral circuitsmay be on top of the memory stack layers rather than on the bottom, andconnected accordingly. Moreover, a prior to the n+ implant step annealto repair damages that were formed during prior step such as layertransfer ion cut related damages and etch step related damages, may beperformed. Furthermore, some of the memory global control lines could beimplemented in the bottom base layer peripheral circuits 702 andconnected to the upper memory structure as it is processed. This couldbe done for the back-bias connections. For example, instead of theback-bias connectivity 752 being formed after the memory stack, the baselayers from the peripheral circuits 702 may provide the back-biasconnection. In such case before depositing the gate electrode material,a contact to those connections would be etched in the soon to be backbias regions 712. Many other modifications within the scope of thepresent invention will suggest themselves to such skilled persons afterreading this specification. Thus the invention is to be limited only bythe appended claims.

The impact ionization efficiency of the polysilicon collector in thearchitecture illustrated in FIGS. 7A-7D may typically be low (the (M−1)factor is typically less than 10⁻³). As a result, a large current may berequired in the holding operation. Using FIG. 6 and its description as aguide, to further improve the efficiency of the holding operation, thesource region (16 of FIG. 6) or the drain region (18 of FIG. 6), whichmay be either of the n+ regions 728 on the sides of p− channel regions724, may be used as the collector region/node and the polysiliconregion, back bias region 712, may now act as the emitter region/node.The holding operation is still governed by the n-p-n bipolar transistorformed by one of the source/drain regions 16 or 18 (n+ regions 728), thefloating body region 24 (p− channel regions 724), and the polysiliconregion 12 (back bias region 712). The back-bias node employed in theholding operation may now be selected from one of the source/drainregions 16 or 18 (n+ regions 728). The terminals used for accessing(i.e. reading and writing) the memory cell are now connected to thepolysilicon region 12 (back bias region 712), the other source/drainregion 16 or 18 (n+ regions 728) not used as the back-bias region, andthe gate electrode 60 (gate regions 760).

As illustrated in FIG. 7K, a dual port RAM may be constructed utilizingmany of the concepts and flow of FIGS. 7A-7J. The processing may proceedto generate the structure of FIG. 7F (with some extra removal areas ofthe perforated gate oxide). At this point, the flow and processingdescribed related to FIG. 7G may be utilized, but the lithographicpattern to form the n+ implanted regions and the gate regions, back-biasregions, and mid-FB junction connection will be different. Using theremaining photoresist of the gate structure regions lithography,portions of p− regions 721 not covered by the gate structure regionsphotoresist may be implanted to form n+ regions 728, and thus form acommon p− channel region 724 that not only includes the region directlybetween gate regions 760 and back-bias regions 712, but also betweenthose regions so to form a commoned p-region (indicated on the topmostsilicon layer in the drawing where visible), which will be the commonfloating body. Additionally, the photolithography pattern may includecovering the gate fill area which may become mid-FB junction connection792. Mid-FB junction connection 792 may directly connect to the commonfloating body p− channel region 724 in the common location (between thegate/body-bias pairs) as that side wall face may be opened up in theearlier formation of perforated gate dielectric 762 at gate dielectricopenings 711 (in a similar fashion as the back-bias regions 712). Gateconnectivity, back-bias connectivity, mid-FB junction connection, and BLconnectivity may then be formed using well known techniques to thoseskilled in the art. Mid-FB junction connection 792 may include anopposite conductivity type doping than that of common floating body p−channel region 724 so to form a junction connection at the related gatedielectric opening 711.

FIG. 7L is a top view exemplary illustration of the dual port RAMfloating body transistors sharing on one side of the Si/SiO₂ stacks (n+regions 728 and commoned floating body p− channel regions 724) gateregions 760 and on the other side of the Si/SiO₂ stacks back biasregions 712 and mid-FB junction connection 792 connected to commonedfloating body p− channel regions 724 in the openings of the perforatedgate dielectric layer 762. The back-bias regions act as the collector ofthe bipolar charging transistor as illustrated in FIG. 4B.

As illustrated in FIG. 7M, a dual port RAM with integrated selecttransistors may be constructed utilizing many of the concepts and flowof FIGS. 7A-7L. The processing may proceed to generate the structure ofFIG. 7F (with some extra removal areas of the perforated gate oxide). Atthis point, the flow and processing described related to FIG. 7G may beutilized, but the lithographic pattern to form the n+ implanted regionsand the gate regions, back-bias regions, select gates, and mid-FBjunction connection will be different. Using the remaining photoresistof the gate structure regions lithography, portions of p− regions 721not covered by the gate structure regions photoresist may be implantedto form n+ regions 728, and thus form a common p− channel region 724that not only includes the region directly between gate regions 760 andback-bias regions 712, but also between those regions so to form acommoned p-region (indicated on the topmost silicon layer in the drawingwhere visible), which may be the common floating body. The p− channelregions of the select transistor gates 796 may also be masked off fromthe implants. Additionally, the photolithography pattern may includecovering the gate fill area which will become mid-FB junction connection792. Mid-FB junction connection 792 may directly connect to the p−channel region 724 in the common location (between the gate/body-biaspairs) as that side wall face is opened up in the earlier formation ofperforated gate dielectric 762 at gate dielectric openings 711 (in asimilar fashion as the back-bias regions 712). The select gates may havethe perforated gate dielectric 762 between the gate and the channel.Gate connectivity, back-bias connectivity, select gate connections,mid-FB junction connection, and BL connectivity may then be formed usingwell known techniques to those skilled in the art. Mid-FB junctionconnection 792 may include an opposite conductivity type doping thanthat of common floating body p− channel region 724 so to form a junctionconnection at the related gate dielectric opening 711.

FIG. 7N is a top view exemplary illustration of the dual port RAM withintegrated select transistors wherein the floating body transistors aresharing on one side of the Si/SiO₂ stacks (n+ regions 728 and floatingbody p− channel regions 724) gate regions 760 and on the other side ofthe Si/SiO₂ stacks back bias regions 712 and mid-FB junction connection792 connected to commoned p− channel regions 724 in the openings of theperforated gate dielectric layer 762. The back-bias regions act as thecollector of the bipolar charging transistor as illustrated in FIG. 4B.The select gates 796, double gated around the associated channel regionof p− channel regions 724, may be utilized to control access to thefloating body transistor on that level and region of the stack.

As illustrated in FIG. 8, another embodiment of the 3D RAM architecturehaving two stable states wherein in the top mono-crystalline layer atleast one of the two devices within the source lines of that layer mayinclude an access device. Access transistor 820 may be connected inseries to the floating body transistor selected to achieve an improvedread and write access time. Second access transistor 821 may also beutilized in parallel with access transistor 820 for more access currentwhen connected to the BL 878 as shown, or may function as anotherfloating body device bit if connected to its own SL (not shown). Theconstruction of the architecture may be similar to the structuredescribed in FIGS. 7A-7J, with modifications described below.

Floating body transistors 870, 871, 872, and 873 may overlay each otherand be connected to corresponding Select or Source Lines 850, 851, 852,and 853 respectively. The select lines may be formed by the N+mono-crystalline layers of source lines (SL) 756 as described in FIG. 7.Overlying the floating body transistors may be access transistor 820which may be connected to the floating body transistors, such asfloating body transistors 873, 872, 871, 870, by the deep contactconnection 836. Deep contact connection 836 may be the deep contactportion of bit line connectivity 754 described in FIG. 7I. Accesstransistor 820 may share the same gate connection, Word Line 842 as thefloating body transistors on the stack below it, such as floating bodytransistors 870, 871, 872, and 873. Floating body transistors 870, 871,872, and 873 may be selected by their source line connections Select orSource Lines 850, 851, 852, and 853 respectively. As well, floating bodytransistors 874, 875, 876, and 877 may be selected by theircorresponding source line connections and accessed by the bit line 878thru access transistor 820, second access transistor 821, or acombination of both access transistor. The read and write signal to theselected floating body may be transferred via the serially connectedaccess transistor, such as access transistor 820, resulting in a shorterwrite or read cycle.

During the read operation, the access transistors of the unselected wordlines are turned off, for example by applying 0.0 volt. As a result, theleakage path due to the unselected memory cells in the unselected wordlines are prevented by being disconnected from the bit line 878. Theleakage currents, if the leakage path is not disconnected, in a largedensity memory array (as expected from a 3D architecture) may be largeenough to slow down or even disrupt the sensing operation of the senseamplifier.

Persons of ordinary skill in the art will appreciate that theillustration in FIG. 8 is exemplary only and is not drawn to scale. Suchskilled persons will further appreciate that many variations arepossible such as, for example, the access transistors could beconstructed on the first mono-crystalline layer (layer closest to theperipheral circuits) and directly connected to the bit line control fromthe peripheral circuits below. Furthermore, by rearranging the stackedstructure single mask layer etch layout geometry, a separate accessdevice and floating body device on each layer could be constructed andaccessed. Many other modifications within the scope of the presentinvention will suggest themselves to such skilled persons after readingthis specification. Thus the invention is to be limited only by theappended claims.

Forming the peripheral circuitry on top of (or ‘above’) the memory stackfabric is an additional embodiment. For example, the techniquesillustrated in FIG. 22F herein illustrate the formation of peripheralcircuits 2298 on top of the memory fabric. Such could be used for thememory fabric described in relation to FIG. 7A to FIG. 7J. In some casesit might be desirable to have peripheral circuits both underneath thememory fabric and above it, using techniques describe herein or in theother patents incorporated by reference. An additional embodimentincludes wherein the memory fabric is first built on an SOI wafer as hasbeen described in respect to illustrations of FIG. 19 and FIG. 20herein. The 3D memory fabric may be first formed and both top and bottomsurfaces could be used for a deep connection etch and both surfacescould be used to form the memory control lines such as bit-lines, andthen the peripheral circuits could be formed on one of both surfacesutilizing the 3D techniques described here and or in the patentsincorporated herein.

An important advantage of the two stable states memory is in respect tolow power standby operation. When the device is placed into standby, theback bias voltage could be dropped to about 50% of Vcc, for example,about 0.8 volt. Other combinatorial circuits could be disconnected fromthe power for further reduction of power consumption. As normaloperation resumes, the voltage of the power supply and that of theback-bias would be brought up to standard levels and the circuit couldresume normal operation. This is one way in which power consumptioncould be reduced without losing the data stored in the memory.

In some cases it might be advantageous to use laser or other annealingtechniques to further improve the quality of the polysilicon byre-crystallization. Another alternative is to deposit the polysiliconwith no doping or p type doping, and then perform an implant step todope the polysilicon with N doping at the desired depth. Depth controlof ion implantation is quite accurate which could be an importantadvantage for very thin layers and stacks. Activation could be donebefore low melting point metals, such as, for example, copper andaluminum, get deposited, and activation could be done for many layerssimultaneously as discussed before. Use of optical anneal techniques toactivate dopants in the Si/SiO2 memory stack may also be employed tominimize damage to the underlying metallization.

As illustrated in FIG. 9, an embodiment wherein a two stable statesmemory may be formed with the back bias region that is formed in thesame mono-crystalline layer/region as the floating body channel. Theback bias region 912 may be spaced apart from the source region 916 andthe drain region 918, and lie in the same mono-crystalline layer 908.The holding operation is performed through the n-p-n bipolar transistorformed by the source region 916, floating body region 924, and the backbias region 912. In this embodiment, the back bias region 912 is formedin the same mono-crystalline layer that includes the floating bodytransistor (formed by the source region 916, drain region 918, and thegate region 960 with gate dielectric 962), wherein the floating bodyregion 24 may be storing the charge. Gate region 960 (including gateconnectivity 970) and gate dielectric 962 are shown in dashes indicatingshadowed, wherein they both overlap and form side gates on bothillustrated layers of mono-crystalline material. In this illustration,mono-crystalline layer 908 may have a 3-side gate and themono-crystalline layer below a two sided gate. The mono-crystallinelayers may be isolated by oxide dielectric layers 930 and isolationdielectric 906 which may be utilized to isolate, and form bonding, ofthe memory stack to the peripheral circuits 902. Drain regions 918 maybe commoned by deep drain contact 974 and access to the source regions916 may be accomplished by source connectivity 972. Back-bias regions912 may be accessed by bb connectivity 976.

Section 2: Monolithic 3D Resistance-Based Memory

While many of today's memory technologies rely on charge storage,several companies are developing non-volatile memory technologies basedon resistance of a material changing. Examples of these resistance-basedmemories include phase change memory, Metal Oxide memory, resistive RAM(RRAM), memristors, solid-electrolyte memory, ferroelectric RAM, MRAM,etc. Background information on these resistive-memory types is given in“Overview of candidate device technologies for storage-class memory,”IBM Journal of Research and Development, vol. 52, no. 4.5, pp. 449-464,July 2008 by Burr, G. W.; Kurdi, B. N.; Scott, J. C.; Lam, C. H.;Gopalakrishnan, K.; Shenoy, R. S. Typical RRAM materials may includetransition metal oxides such as TiOx, NiOx, HFOx, WoX, TaOx, VoX, CuOx,SrToOx, CuSiOx, SiOx, TiON, and electrodes may include Pt, TiN/Ti, TiN,Ru, Ni, W, TaN, Ir, Au. STT-MRAM materials may include Ir, PtMn, CoFe,Ru, CoFeB, MgO, CoFeB, Ta.

FIGS. 10A-J describe a novel memory architecture for resistance-basedmemories, and a procedure for its construction. The memory architectureutilizes junction-less transistors and has a resistance-based memoryelement in series with a transistor selector. No mask is utilized on a“per-memory-layer” basis for the monolithic 3D resistance change memory(or resistive memory) concept shown in FIG. 10A-J, and all other masksmay be shared between different layers. The process flow may includeseveral steps that occur in the following sequence.

Step (A): Peripheral circuits 1002 may be first constructed and abovethis an insulator layer, such as a layer of silicon dioxide, oxide layer1004 may be deposited. FIG. 10A shows a drawing illustration after Step(A).Step (B): FIG. 10B illustrates the structure after Step (B). A wafer ofn+ Silicon 1008 may have an oxide layer 1006 grown or deposited aboveit. Following this, hydrogen may be implanted into the n+ Silicon waferat a certain depth indicated by 1014. Alternatively, some other atomicspecies such as Helium could be (co-)implanted. This hydrogen implantedn+ Silicon wafer 1008 may form the top layer 1010. The bottom layer 1012may include the peripheral circuits 1002 with oxide layer 1004. The toplayer 1010 may be flipped and bonded to the bottom layer 1012 usingoxide-to-oxide bonding.Step (C): FIG. 10C illustrates the structure after Step (C). The stackof top and bottom wafers after Step (B) may be cleaved at the hydrogenplane 1014 using either a anneal or a sideways mechanical force or othermeans. A CMP process may then conducted. A layer of silicon oxide 1018may then be deposited atop the n+ Silicon layer 1016. At the end of thisstep, a single-crystal n+ Si layer 1016 exists atop the peripheralcircuits, and this has been achieved using layer-transfer techniques.Step (D): FIG. 10D illustrates the structure after Step (D). Usingmethods similar to Step (B) and (C), multiple n+ silicon layers 1020 maybe formed with silicon oxide layers in between. The composition of thesilicon oxide regions within the stacked Si/SiO₂ layers may beinsulators or dielectrics other than silicon dioxide such as, forexample, a low-k dielectric, carbon containing silicon oxides, amorphouscarbon. The thickness of the silicon oxide regions within the stackedSi/SiO₂ layers may be adjusted to minimize layer to layer, strata tostrata disturb mechanisms, and may include thicknesses of 50 nm, 75 nm,100 nm, 150 nm, 200 nm, 500 nm, and less than 1000 nm. A thin conductivelayer, such as a metal, may also be formed between the two bondingoxides to form a field shield to mitigate layer to layer, strata tostrata disturb mechanisms, and may be electrically floating or tied to abias, such as ground or Vdd. The stacked Si/SiO₂ layers mayalternatively be formed by successive ion implants of oxygen atoms/ionsto various depths from the top surface of a mono-crystalline siliconwafer/substrate and then heat treated to form oxide layers thus formingsilicon layers in-between the oxide layers, a layered ‘SIMOX’ processapproach.Step (E): FIG. 10E illustrates the structure after Step (E). Lithographyand etch processes may then be utilized to make a structure as shown inthe figure.Step (F): FIG. 10F illustrates the structure after Step (F). Gatedielectric 1026 and gate electrode 1024 may then be deposited followingwhich a CMP may be performed to planarize the gate electrode 1024regions. Lithography and etch may be utilized to define gate regions.Gate dielectric 1026 and gate electrode 1024 may be, for example, a HKMGstructure or a TEL SPA radical oxidation oxide and an appropriate workfunction electrode, for example, tungsten, degenerately dopedpolysilicon or amorphous silicon. Although the width of the n+ regions1021 near and under the gates are illustrated as being larger than thethickness, one skilled in the art would recognize that the reverse(i.e., thickness larger than width) could be formed to provide increasedgate control of the transistor channel.Step (G): FIG. 10G illustrates the structure after Step (G). A siliconoxide layer 1030 may then be deposited and planarized. The silicon oxidelayer is shown transparent in the figure for clarity, along withword-line (WL) 1032 and source-line (SL) 1034 regions.Step (H): FIG. 10H illustrates the structure after Step (H). Vias may beetched through multiple layers of silicon and silicon dioxide as shownin the figure. A resistance change memory material 1036 may then bedeposited (preferably with atomic layer deposition (ALD)). Examples ofsuch a material include hafnium oxide, well known to change resistanceby applying voltage. An electrode for the resistance change memoryelement may then be deposited (preferably using ALD) and is shown aselectrode/BL contact 1040. A CMP process may then be conducted toplanarize the surface. It can be observed that multiple resistancechange memory elements in series with junctionless transistors arecreated after this step.Step (I): FIG. 10I illustrates the structure after Step (I). BLs 1038may then be constructed. Contacts may be made to BLs, WLs and SLs of thememory array at its edges. SL contacts can be made into stair-likestructures using techniques described in “Bit Cost Scalable Technologywith Punch and Plug Process for Ultra High Density Flash Memory,” VLSITechnology, 2007 IEEE Symposium on, vol., no., pp. 14-15, 12-14 Jun.2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., followingwhich contacts can be constructed to them. Formation of stair-likestructures for SLs could be achieved in steps prior to Step (I) as well.

FIG. 10J shows cross-sectional views of the array for clarity.

A 3D resistance change memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in transistor channels, (2) some of the memorycell control lines, e.g., source-lines SL, constructed of heavily dopedsilicon and embedded in the memory cell layer, (3) side gates that aresimultaneously deposited over multiple memory layers for transistors,and (4) monocrystalline (or single-crystal) silicon layers obtained bylayer transfer techniques such as ion-cut.

FIG. 11A-K describe an alternative process flow to construct ahorizontally-oriented monolithic 3D resistive memory array. Thisembodiment has a resistance-based memory element in series with atransistor selector. No mask is utilized on a “per-memory-layer” basisfor the monolithic 3D resistance change memory (or resistive memory)concept shown in FIG. 11A-K, and all other masks may be shared betweendifferent layers. The process flow may include several steps asdescribed in the following sequence.

Step (A): Peripheral circuits with tungsten wiring 1102 may be firstconstructed and above this a layer of silicon dioxide 1104 is deposited.FIG. 11A shows a drawing illustration after Step (A).Step (B): FIG. 11B illustrates the structure after Step (B). A wafer ofp− Silicon 1108 may have an oxide layer 1106 grown or deposited aboveit. Following this, hydrogen may be implanted into the p− Silicon waferat a certain depth indicated by 1114. Alternatively, some other atomicspecies such as Helium could be (co-)implanted. This hydrogen implantedp− Silicon wafer 1108 may form the top layer 1110. The bottom layer 1112may include the peripheral circuits 1102 with oxide layer 1104. The toplayer 1110 may be flipped and bonded to the bottom layer 1112 usingoxide-to-oxide bonding.Step (C): FIG. 11C illustrates the structure after Step (C). The stackof top and bottom wafers after Step (B) may be cleaved at the hydrogenplane 1114 using either a anneal or a sideways mechanical force or othermeans. A CMP process may then be conducted. A layer of silicon oxide1118 may then be deposited atop the p− Silicon layer 1116. At the end ofthis step, a single-crystal p− Silicon layer 1116 exists atop theperipheral circuits, and this has been achieved using layer-transfertechniques.Step (D): FIG. 11D illustrates the structure after Step (D). Usingmethods similar to Step (B) and (C), multiple p− silicon layers 1120 maybe formed with silicon oxide layers in between. The composition of thesilicon oxide regions within the stacked Si/SiO₂ layers may beinsulators or dielectrics other than silicon dioxide such as, forexample, a low-k dielectric, carbon containing silicon oxides, amorphouscarbon. The thickness of silicon oxide regions within the stackedSi/SiO₂ layers may be adjusted to minimize layer to layer, strata tostrata disturb mechanisms, and may include thicknesses of 50 nm, 75 nm,100 nm, 150 nm, 200 nm, 500 nm, and less than 1000 nm. A thin conductivelayer, such as a metal, may also be formed between the two bondingoxides to form a field shield to mitigate layer to layer, strata tostrata disturb mechanisms, and may be electrically floating or tied to abias, such as ground or Vdd. The stacked Si/SiO₂ layers mayalternatively be formed by successive ion implants of oxygen atoms/ionsto various depths from the top surface of a mono-crystalline siliconwafer/substrate and then heat treated to form oxide layers thus formingsilicon layers in-between the oxide layers, a layered ‘SIMOX’ processapproach.Step (E): FIG. 11E illustrates the structure after Step (E), includinginsulator regions such as silicon oxide 1122. Lithography and etchprocesses may then be utilized to construct a structure as shown in thefigure.Step (F): FIG. 11F illustrates the structure on after Step (F). Gatedielectric 1126 and gate electrode 1124 may then be deposited followingwhich a CMP may be done to planarize the gate electrode 1124 regions.Lithography and etch may be utilized to define gate regions. Gatedielectric 1126 and gate electrode 1124 may be, for example, a HKMGstructure or a TEL SPA radical oxidation oxide and an appropriate workfunction electrode, for example, tungsten, degenerately dopedpolysilicon or amorphous silicon. Although the width of the p− regions1121 near and under the gates are illustrated as being larger than thethickness, one skilled in the art would recognize that the reverse(i.e., thickness larger than width) could be formed to provide increasedgate control of the transistor channel.Step (G): FIG. 11G illustrates the structure after Step (G). Using thehard mask defined in Step (F), p− regions not covered by the gate may beimplanted to form n+ regions 1128. Spacers may be utilized during thismulti-step implantation process and layers of silicon present indifferent layers of the stack may have different spacer widths toaccount for lateral straggle of buried layer implants. Bottom layerscould have larger spacer widths than top layers. A thermal annealingstep, such as a RTA or spike anneal or laser anneal or flash anneal, maythen be conducted to activate n+ doped regions.Step (H): FIG. 11H illustrates the structure after Step (H). A siliconoxide layer 1130 may then be deposited and planarized. The silicon oxidelayer is shown transparent in the figure for clarity, along withword-line (WL) 1132 and source-line (SL) 1134 regions.Step (I): FIG. 11I illustrates the structure after Step (I). Vias may beetched through multiple layers of silicon and silicon dioxide as shownin the figure. A resistance change memory material 1136 may then bedeposited (preferably with atomic layer deposition (ALD)). Examples ofsuch a material include hafnium oxide, which is well known to changeresistance by applying voltage. An electrode for the resistance changememory element may then be deposited (preferably using ALD) and is shownas electrode/BL contact 1140. A CMP process may then be conducted toplanarize the surface. It can be observed that multiple resistancechange memory elements in series with transistors are created after thisstep.Step (J): FIG. 11J illustrates the structure after Step (J). BLs 1138may be constructed. Contacts may be made to BLs, WLs and SLs of thememory array at its edges. SL contacts can be made into stair-likestructures using techniques described in “Bit Cost Scalable Technologywith Punch and Plug Process for Ultra High Density Flash Memory,” VLSITechnology, 2007 IEEE Symposium on, vol., no., pp. 14-15, 12-14 Jun.2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., followingwhich contacts can be constructed to them. Formation of stair-likestructures for SLs could be done in steps prior to Step (I) as well.

FIG. 11K shows cross-sectional views of the array for clarity.

A 3D resistance change memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in transistor channels, (2) some of the memorycell control lines—e.g., source-lines SL, constructed of heavily dopedsilicon and embedded in the memory cell layer, (3) side gatessimultaneously deposited over multiple memory layers for transistors,and (4) monocrystalline (or single-crystal) silicon layers obtained bylayer transfer techniques such as ion-cut.

FIG. 12A-L describes an alternative process flow to construct ahorizontally-oriented monolithic 3D resistive memory array. Thisembodiment has a resistance-based memory element in series with atransistor selector. One mask is utilized on a “per-memory-layer” basisfor the monolithic 3D resistance change memory (or resistive memory)concept shown in FIG. 12A-L, and all other masks may be shared betweendifferent layers. The process flow may include several steps asdescribed in the following sequence.

Step (A): Peripheral circuits 1202 with tungsten wiring may be firstconstructed and above this a layer of silicon dioxide 1204 may bedeposited. FIG. 12A illustrates the structure after Step (A).Step (B): FIG. 12B illustrates the structure after Step (B). A wafer ofp− Silicon 1206 may have an oxide layer 1208 grown or deposited aboveit. Following this, hydrogen may be implanted into the p− Silicon waferat a certain depth indicated by 1210. Alternatively, some other atomicspecies such as Helium could be (co-)implanted. This hydrogen implantedp− Silicon wafer 1206 may form the top layer 1212. The bottom layer 1214may include the peripheral circuits 1202 with oxide layer 1204. The toplayer 1212 may be flipped and bonded to the bottom layer 1214 usingoxide-to-oxide bonding.Step (C): FIG. 12C illustrates the structure after Step (C). The stackof top and bottom wafers after Step (B) may be cleaved at the hydrogenplane 1210 using either a anneal or a sideways mechanical force or othermeans. A CMP process may be conducted. At the end of this step, asingle-crystal p− Si layer exists atop the peripheral circuits, and thishas been achieved using layer-transfer techniques.Step (D): FIG. 12D illustrates the structure after Step (D). Usinglithography and then implantation, n+ regions 1216 and p− regions 1218may be formed on the transferred layer of p− Si after Step (C).Step (E): FIG. 12E illustrates the structure after Step (E). An oxidelayer 1220 may be deposited atop the structure obtained after Step (D).A first layer of Si/SiO₂ 1222 may be formed atop the peripheral circuitlayer 1202. The composition of the ‘SiO₂’ layer within the stackedSi/SiO₂ layers such as Si/SiO₂ 1222, may be insulators or dielectricsother than silicon dioxide such as, for example, a low-k dielectric,carbon containing silicon oxides, amorphous carbon. The thickness of the‘SiO₂’ insulator layer within the stacked Si/SiO₂ layers such as Si/SiO₂1222 may be adjusted to minimize layer to layer, strata to stratadisturb mechanisms, and may include thicknesses of 50 nm, 75 nm, 100 nm,150 nm, 200 nm, 500 nm, and less than 1000 nm. A thin conductive layer,such as a metal, may also be formed between the two bonding oxides toform a field shield to mitigate layer to layer, strata to strata disturbmechanisms, and may be electrically floating or tied to a bias, such asground or Vdd. The stacked Si/SiO₂ layers, such as Si/SiO₂ 1222, mayalternatively be formed by successive ion implants of oxygen atoms/ionsto various depths from the top surface of a mono-crystalline siliconwafer/substrate and then heat treated to form oxide layers thus formingsilicon layers in-between the oxide layers, a layered ‘SIMOX’ processapproach.Step (F): FIG. 12F illustrates the structure after Step (F). Usingprocedures similar to Steps (B)-(E), additional Si/SiO₂ layers 1224 and1226 may be formed atop Si/SiO₂ layer 1222. A rapid thermal anneal (RTA)or spike anneal or flash anneal or laser anneal may be done to activateimplanted layers 1222, 1224 and 1226 (and possibly also the peripheralcircuit layer 1202). Alternatively, the layers 1222, 1224 and 1226 maybe annealed layer-by-layer as soon as their implantations are done usinga laser anneal system.Step (G): FIG. 12G illustrates the structure after Step (G). Lithographyand etch processes may be utilized to make a structure as shown in thefigure.Step (H): FIG. 12H illustrates the structure after Step (H). Gatedielectric 1228 and gate electrode 1230 may be deposited following whicha CMP may be done to planarize the gate electrode 1230 regions.Lithography and etch may be utilized to define gate regions over the p−silicon regions (eg. p− Si region 1218 after Step (D)). Note that gatewidth could be slightly larger than p− region width to compensate foroverlay errors in lithography. Gate dielectric 1228 and gate electrode1230 may be, for example, a HKMG structure or a TEL SPA radicaloxidation oxide and an appropriate work function electrode, for example,tungsten, degenerately doped polysilicon or amorphous silicon. Althoughthe width of the p− regions 1217 and n+ regions 1215 near and under thegates are illustrated as being larger than the thickness, one skilled inthe art would recognize that the reverse (i.e., thickness larger thanwidth) could be formed to provide increased gate control of thetransistor channel.Step (I): FIG. 12I illustrates the structure after Step (I). A siliconoxide layer 1232 may be deposited and planarized. It is showntransparent in the figure for clarity. Word-line (WL) and Source-line(SL) regions are shown in the figure.Step (J): FIG. 12J illustrates the structure after Step (J). Vias may beetched through multiple layers of silicon and silicon dioxide as shownin the figure. A resistance change memory material 1236 may be deposited(preferably with atomic layer deposition (ALD)). Examples of such amaterial include hafnium oxide, which is well known to change resistanceby applying voltage. An electrode for the resistance change memoryelement may be deposited (preferably using ALD) and is shown aselectrode/BL contact 1240. A CMP process may be conducted to planarizethe surface. It can be observed that multiple resistance change memoryelements in series with transistors are created after this step.Step (K): FIG. 12K illustrates the structure after Step (K). BLs 1236may be constructed. Contacts may be made to BLs 1238, WLs 1232 and SLs1234 of the memory array at its edges. SL contacts can be made intostair-like structures using techniques described in “Bit Cost ScalableTechnology with Punch and Plug Process for Ultra High Density FlashMemory,” VLSI Technology, 2007 IEEE Symposium on, vol., no., pp. 14-15,12-14 Jun. 2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; etal., following which contacts can be constructed to them. Formation ofstair-like structures for SLs could be achieved in steps prior to Step(J) as well.

FIG. 12L shows cross-sectional views of the array for clarity.

A 3D resistance change memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in transistor channels, (2) some of the memorycell control lines, e.g., source-lines SL, constructed of heavily dopedsilicon and embedded in the memory cell layer, (3) side gatessimultaneously deposited over multiple memory layers for transistors,and (4) monocrystalline (or single-crystal) silicon layers obtained bylayer transfer techniques such as ion-cut.

FIG. 13A-F describes an alternative process flow to construct ahorizontally-oriented monolithic 3D resistive memory array. Thisembodiment has a resistance-based memory element in series with atransistor selector. Two masks are utilized on a “per-memory-layer”basis for the monolithic 3D resistance change memory (or resistivememory) concept shown in FIG. 13A-F, and all other masks may be sharedbetween different layers. The process flow may include several steps asdescribed in the following sequence.

Step (A): The process flow may start with a p− silicon wafer 1302 withan oxide coating 1304. FIG. 13A illustrates the structure after Step(A).Step (B): FIG. 13B illustrates the structure after Step (B). Using aprocess flow similar to FIG. 12B, a portion of the p− silicon layer 1302may be transferred atop a layer of peripheral circuits 1306. Theperipheral circuits 1306 preferably use tungsten wiring.Step (C): FIG. 13C illustrates the structure after Step (C). Isolationregions for transistors may be formed using a shallow-trench-isolation(STI) process. Following this, a gate dielectric 1310 and a gateelectrode 1308 may be deposited. Gate dielectric 1310 and gate electrode1308 may be, for example, a HKMG structure or a TEL SPA radicaloxidation oxide and an appropriate work function electrode, for example,tungsten, degenerately doped polysilicon or amorphous silicon.Step (D): FIG. 13D illustrates the structure after Step (D). The gatemay be patterned, and source-drain regions 1312 may be formed byimplantation. An inter-layer dielectric (ILD) 1314 may be also formed.Step (E): FIG. 13E illustrates the structure after Step (E). Using stepssimilar to Step (A) to Step (D), a second layer of transistors 1316 maybe formed above the first layer of transistors 1314. An RTA or someother type of anneal, such as the optical anneals described herein, maybe performed to activate dopants in the memory layers (and potentiallyalso the peripheral transistors).Step (F): FIG. 13F illustrates the structure after Step (F). Vias may beetched through multiple layers of silicon and silicon dioxide as shownin the figure. A resistance change memory material 1322 may be deposited(preferably with atomic layer deposition (ALD)). Examples of such amaterial include hafnium oxide, which is well known to change resistanceby applying voltage. An electrode for the resistance change memoryelement may be deposited (preferably using ALD) and is shown aselectrode 1326. A CMP process may be conducted to planarize the surface.Contacts are made to drain terminals of transistors in different memorylayer as well. Note that gates of transistors in each memory layer maybe connected together perpendicular to the plane of the figure to formword-lines (WL 1320). Wiring for bit-lines (BL 1318) and source-lines(SL 1324) may be constructed. Contacts may be made between BLs, WLs andSLs with the periphery at edges of the memory array. Multiple resistancechange memory elements in series with transistors may be created afterthis step.

A 3D resistance change memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in the transistor channels, and (2)monocrystalline (or single-crystal) silicon layers obtained by layertransfer techniques such as ion-cut.

While explanations have been given for formation of monolithic 3Dresistive memories with ion-cut in this section, it is clear to oneskilled in the art that alternative implementations are possible. BL andSL nomenclature has been used for two terminals of the 3D resistivememory array, and this nomenclature can be interchanged. Moreover,selective epi technology or laser recrystallization technology could beutilized for implementing structures shown in FIG. 10A-J, FIG. 11A-K,FIG. 12A-L and FIG. 13A-F. Various other types of layer transfer schemesthat have been described herein and in incorporated patent referencescan be utilized for construction of various 3D resistive memorystructures. One could also use buried wiring, i.e. where wiring formemory arrays is below the memory layers but above the periphery. Othervariations of the monolithic 3D resistive memory concepts are possible.

Section 3: Monolithic 3D Charge-Trap Memory

While resistive memories described previously form a class ofnon-volatile memory, others classes of non-volatile memory exist. NANDflash memory forms one of the most common non-volatile memory types. Itcan be constructed of two main types of devices: floating-gate deviceswhere charge is stored in a floating gate and charge-trap devices wherecharge is stored in a charge-trap layer such as Silicon Nitride.Background information on charge-trap memory can be found in “IntegratedInterconnect Technologies for 3D Nanoelectronic Systems”, Artech House,2009 by Bakir and Meindl (“Bakir”) and “A Highly Scalable 8-Layer 3DVertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried ChannelBE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue,et al. The architectures shown in FIG. 14A-F, FIG. 15A-G and FIG. 16A-Dare relevant for any type of charge-trap memory.

FIG. 14A-F describes a process flow to construct a horizontally-orientedmonolithic 3D charge trap memory. Two masks are utilized on a“per-memory-layer” basis for the monolithic 3D charge trap memoryconcept shown in FIG. 14A-F, while other masks may be shared between allconstructed memory layers. The process flow may include several steps,which may occur in the following sequence.

Step (A): A p− Silicon wafer 1402 may be taken and an oxide layer 1404may be grown or deposited above it. FIG. 14A illustrates the structureafter Step (A).Step (B): FIG. 14B illustrates the structure after Step (B). Using aprocedure similar to the one shown in FIG. 12B, a portion of the p− Siwafer 1402, such as p− silicon layer 1403, may be transferred atop aperipheral circuit layer 1406. The periphery may be designed such thatit can withstand the RTA or optical anneals for activating dopants inthe memory layers formed atop it.Step (C): FIG. 14C illustrates the structure after Step (C). Isolationregions may be formed in the p− Si layer 1403 atop the peripheralcircuit layer 1406. This lithography step and all future lithographysteps may be formed with good alignment to features on the peripheralcircuit layer 1406 since the p− silicon layer 1403 is thin andreasonably transparent to the lithography tool. A dielectric layer 1410(eg. Oxide-nitride-oxide ONO layer) may be deposited following which agate electrode layer 1408 (eg. polysilicon) may be deposited.Step (D): FIG. 14D illustrates the structure after Step (D). The gateregions deposited in Step (C) may be patterned and etched. Followingthis, source-drain regions 1412 may be implanted. An inter-layerdielectric 1414 may be deposited and planarized.Step (E): FIG. 14E illustrates the structure after Step (E). Usingprocedures similar to Step (A) to Step (D), another layer of memory,such as a second NAND string 1416, may be formed atop the first NANDstring 1414.Step (F): FIG. 14F illustrates the structure after Step (F). Contacts1418 may be made to connect bit-lines (BL) and source-lines (SL) to theNAND string. Contacts to the well of the NAND string may be made. Allthese contacts could be constructed of heavily doped polysilicon or someother material. An anneal to activate dopants in source-drain regions oftransistors in the NAND string (and potentially also the periphery) maybe conducted. Following this, construction of wiring layers for thememory array may be conducted.

A 3D charge-trap memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in transistor channels, and (2) monocrystalline(or single-crystal) silicon layers obtained by layer transfer techniquessuch as ion-cut. This use of monocrystalline silicon (or single crystalsilicon) using ion-cut can be a key differentiator for some embodimentsof the current invention vis-à-vis prior work.

FIG. 15A-G describes a memory architecture for single-crystal 3Dcharge-trap memories, and a procedure for its construction. It utilizesjunction-less transistors. No mask is utilized on a “per-memory-layer”basis for the monolithic 3D charge-trap memory concept shown in FIG.15A-G, and all other masks are shared between different layers. Theprocess flow may include several steps as described in the followingsequence.

Step (A): Peripheral circuits 1502 may be constructed and above this alayer of silicon dioxide 1504 may be deposited. FIG. 15A shows a drawingillustration after Step (A).Step (B): FIG. 15B illustrates the structure after Step (B). A wafer ofn+ Silicon 1508 may have an oxide layer 1506 grown or deposited aboveit. Following this, hydrogen may be implanted into the n+ Silicon waferat a certain depth indicated by 1514. Alternatively, some other atomicspecies such as Helium could be implanted. This hydrogen implanted n+Silicon wafer 1508 may form the top layer 1510. The bottom layer 1512may include the peripheral circuits 1502 with oxide layer 1504. The toplayer 1510 may be flipped and bonded to the bottom layer 1512 usingoxide-to-oxide bonding.Step (C): FIG. 15C illustrates the structure after Step (C). The stackof top and bottom wafers after Step (B) may be cleaved at the hydrogenplane 1514 using either a anneal or a sideways mechanical force or othermeans. A CMP process may be conducted. A layer of silicon oxide 1518 maybe deposited atop the n+ Silicon layer 1516. At the end of this step, asingle-crystal n+ Si layer 1516 exists atop the peripheral circuits, andthis has been achieved using layer-transfer techniques.Step (D): FIG. 15D illustrates the structure after Step (D). Usingmethods similar to Step (B) and (C), multiple n+ silicon layers 1520 maybe formed with silicon oxide layers in between. The composition of the‘SiO₂’ layer within the stacked Si/SiO₂ layers may be insulators ordielectrics other than silicon dioxide such as, for example, a low-kdielectric, carbon containing silicon oxides, amorphous carbon. Thethickness of the ‘SiO₂’ insulator layer within the stacked Si/SiO₂layers may be adjusted to minimize layer to layer, strata to stratadisturb mechanisms, and may include thicknesses of 50 nm, 75 nm, 100 nm,150 nm, 200 nm, 500 nm, and less than 1000 nm. A thin conductive layer,such as a metal, may also be formed between the two bonding oxides toform a field shield to mitigate layer to layer, strata to strata disturbmechanisms, and may be electrically floating or tied to a bias, such asground or Vdd. The stacked Si/SiO₂ layers may alternatively be formed bysuccessive ion implants of oxygen atoms/ions to various depths from thetop surface of a mono-crystalline silicon wafer/substrate and then heattreated to form oxide layers thus forming silicon layers in-between theoxide layers, a layered ‘SIMOX’ process approach.Step (E): FIG. 15E illustrates the structure after Step (E). Lithographyand etch processes may be utilized to make a structure as shown in thefigure.Step (F): FIG. 15F illustrates the structure after Step (F). Gatedielectric 1526 and gate electrode 1524 may be deposited following whicha CMP may be done to planarize the gate electrode 1524 regions.Lithography and etch may be utilized to define gate regions. Gates ofthe NAND string 1536 as well as gates of select gates of the NAND string1538 may be defined. Gate dielectric 1526 and gate electrode 1524 maybe, for example, a HKMG structure or a TEL SPA radical oxidation oxideand an appropriate work function electrode, for example, tungsten,degenerately doped polysilicon or amorphous silicon. Although the widthof the n+ regions near and under the gates are illustrated as beinglarger than the thickness, one skilled in the art would recognize thatthe reverse (i.e., thickness larger than width) could be formed toprovide increased gate control of the transistor channel.Step (G): FIG. 15G illustrates the structure after Step (G). A siliconoxide layer 1530 may be deposited and planarized. It is showntransparent in the figure for clarity. Word-lines, bit-lines andsource-lines may be defined as shown in the figure, including wiring forthe select gates 1532 and cell source regions 1534 for connection to thecell sources. Contacts may be formed to various regions/wires at theedges of the array as well. SL contacts can be made into stair-likestructures using techniques described in “Bit Cost Scalable Technologywith Punch and Plug Process for Ultra High Density Flash Memory,” VLSITechnology, 2007 IEEE Symposium on, vol., no., pp. 14-15, 12-14 Jun.2007 by Tanaka, H.; Kido, M.; Yahashi, K.; Oomura, M.; et al., followingwhich contacts can be constructed to them. Formation of stair-likestructures for SLs could be performed in steps prior to Step (G) aswell. A 3D charge-trap memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in transistor channels, (2) some of the memorycell control lines—e.g., bit lines BL, constructed of heavily dopedsilicon and embedded in the memory cell layer, (3) side gatessimultaneously deposited over multiple memory layers for transistors,and (4) monocrystalline (or single-crystal) silicon layers obtained bylayer transfer techniques such as ion-cut. This use of single-crystalsilicon obtained with ion-cut is a key differentiator from past work on3D charge-trap memories such as “A Highly Scalable 8-Layer 3DVertical-Gate (VG) TFT NAND Flash Using Junction-Free Buried ChannelBE-SONOS Device,” Symposium on VLSI Technology, 2010 by Hang-Ting Lue,et al. that used polysilicon.

While FIG. 14A-F and FIG. 15A-G give two examples of how single-crystalsilicon layers with ion-cut can be used to produce 3D charge-trapmemories, the ion-cut technique for 3D charge-trap memory is fairlygeneral. It could be utilized to produce any horizontally-oriented 3Dmonocrystalline-silicon charge-trap memory. FIG. 16A-D furtherillustrate how general the process can be. One or more doped siliconlayers 1602 can be layer transferred atop any peripheral circuit layer1606 using procedures shown in FIG. 12B, including insulator layers suchas oxide 1604. These are indicated in FIG. 16A, FIG. 16B and FIG. 16C.Following this, different procedures can be utilized to form differenttypes of 3D charge-trap memories. For example, procedures shown in “AHighly Scalable 8-Layer 3D Vertical-Gate (VG) TFT NAND Flash UsingJunction-Free Buried Channel BE-SONOS Device,” Symposium on VLSITechnology, 2010 by Hang-Ting Lue, et al. and “Multi-layered VerticalGate NAND Flash overcoming stacking limit for terabit density storage”,Symposium on VLSI Technology, 2009 by W. Kim, S. Choi, et al. can beused to produce the two different types of horizontally oriented singlecrystal silicon 3D charge trap memory shown in FIG. 16D.

Section 4: Monolithic 3D Floating-Gate Memory

While charge-trap memory forms one type of non-volatile memory,floating-gate memory is another type. Background information onfloating-gate NAND flash memory can be found in Bez, R. et al.Introduction to Flash memory. Proc. IEEE 91, 489-502 (2003). There aredifferent types of floating-gate memory based on different materials anddevice structures. The architectures shown in FIG. 17A-F and FIG. 18A-Hare relevant for any type of floating-gate memory.

FIG. 17A-F describe a process flow to construct a horizontally-orientedmonolithic 3D floating-gate memory. Two masks are utilized on a“per-memory-layer” basis for the monolithic 3D floating-gate memoryconcept shown in FIG. 17A-F, while other masks may be shared between allconstructed memory layers. The process flow may include several steps asdescribed in the following sequence.

Step (A): A p− Silicon wafer 1702 may be taken and an oxide layer 1704may be grown or deposited above it. FIG. 17A illustrates the structureafter Step (A).Step (B): FIG. 17B illustrates the structure after Step (B). Using aprocedure similar to the one shown in FIG. 12B, a portion of the p− Siwafer 1702, such as p− silicon layer 1703, may be transferred atop aperipheral circuit layer 1706. The periphery may be designed such thatit can withstand the RTA or optical annealing techniques for activatingdopants in memory layers formed atop it.Step (C): FIG. 17C illustrates the structure after Step (C). Afterdeposition of the tunnel oxide 1710 and floating gate 1708, isolationregions may be formed in the p− silicon layer 1703 atop the peripheralcircuit layer 1706. This lithography step and all future lithographysteps may be formed with good alignment to features on the peripheralcircuit layer 1706 since the p− silicon layer 1703 is thin andreasonably transparent to the lithography tool.Step (D): FIG. 17D illustrates the structure after Step (D). Aninter-poly-dielectric (IPD) layer (eg. Oxide-nitride-oxide ONO layer)may be deposited following which a control gate electrode 1720 (eg.polysilicon) may be deposited. The gate regions deposited in Step (C)may be patterned and etched. Following this, source-drain regions 1712may be implanted. An inter-layer dielectric 1714 may be deposited andplanarized.Step (E): FIG. 17E illustrates the structure after Step (E). Usingprocedures similar to Step (A) to Step (D), another layer of memory, asecond NAND string 1716, may be formed atop the first NAND string 1714.Step (F): FIG. 17F illustrates the structure after Step (F). Contacts1718 may be made to connect bit-lines (BL) and source-lines (SL) to theNAND string. Contacts to the well of the NAND string may be made. Allthese contacts could be constructed of heavily doped polysilicon or someother material. An anneal to activate dopants in source-drain regions oftransistors in the NAND string (and potentially also the periphery) maybe conducted. Following this, wiring layers for the memory array may beconstructed.

A 3D floating-gate memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flow in substantially thehorizontal direction in transistor channels, (2) monocrystalline (orsingle-crystal) silicon layers obtained by layer transfer techniquessuch as ion-cut. This use of monocrystalline silicon (or single crystalsilicon) using ion-cut is a key differentiator for some embodiments ofthe current invention vis-à-vis prior work. Past work used selective epitechnology or laser recrystallization or polysilicon.

FIG. 18A-H show a novel memory architecture for 3D floating-gatememories, and a procedure for its construction. The memory architectureutilizes junction-less transistors. One mask is utilized on a“per-memory-layer” basis for the monolithic 3D floating-gate memoryconcept shown in FIG. 18A-H, and all other masks may be shared betweendifferent layers. The process flow may include several steps that may bedescribed in the following sequence.

Step (A): Peripheral circuits 1802 may be constructed and above this alayer of silicon dioxide 1804 may be deposited. FIG. 18A illustrates thestructure after Step (A).Step (B): FIG. 18B illustrates the structure after Step (B). A wafer ofn+ Silicon 1808 may have an oxide layer 1806 grown or deposited aboveit. Following this, hydrogen may be implanted into the n+ Silicon waferat a certain depth indicated by 1814. Alternatively, some other atomicspecies such as Helium could be implanted. This hydrogen implanted n+Silicon wafer 1808 may form the top layer 1810. The bottom layer 1812may include the peripheral circuits 1802 with oxide layer 1804. The toplayer 1810 may be flipped and bonded to the bottom layer 1812 usingoxide-to-oxide bonding.Step (C): FIG. 18C illustrates the structure after Step (C). The stackof top and bottom wafers after Step (B) may be cleaved at the hydrogenplane 1814 using either a anneal or a sideways mechanical force or othermeans. A CMP process may be conducted. At the end of this step, asingle-crystal n+ Si layer 1816 exists atop the peripheral circuits, andthis has been achieved using layer-transfer techniques.Step (D): FIG. 18D illustrates the structure after Step (D). Usinglithography and etch, the n+ silicon layer 1807 may be defined.Step (E): FIG. 18E illustrates the structure after Step (E). A tunneloxide layer 1808 may be grown or deposited following which a polysiliconlayer 1810 for forming future floating gates may be deposited. A CMPprocess may be conducted.Step (F): FIG. 18F illustrates the structure after Step (F). Usingsimilar procedures, multiple levels of memory may be formed with oxidelayers in between.Step (G): FIG. 18G illustrates the structure after Step (G). Thepolysilicon region for floating gates 1810 may be etched to form thepolysilicon region 1811.Step (H): FIG. 18H illustrates the structure after Step (H). Inter-polydielectrics (IPD) 1812 and control gates 1814 may be deposited andpolished.

While the steps shown in FIG. 18A-H describe formation of a few floatinggate transistors, it will be obvious to one skilled in the art that anarray of floating-gate transistors can be constructed using similartechniques and well-known memory access/decoding schemes.

A 3D floating-gate memory has thus been constructed, with (1)horizontally-oriented transistors—i.e. current flowing in substantiallythe horizontal direction in transistor channels, (2) monocrystalline (orsingle-crystal) silicon layers obtained by layer transfer techniquessuch as ion-cut, (3) side gates that are simultaneously deposited overmultiple memory layers for transistors, and (4) some of the memory cellcontrol lines are in the same memory layer as the devices. The use ofmonocrystalline silicon (or single crystal silicon) layer obtained byion-cut in (2) is a key differentiator for some embodiments of thecurrent invention vis-à-vis prior work. Past work used selective epitechnology or laser recrystallization or polysilicon.

Section 5: Alternative Implementations of Various Monolithic 3D MemoryConcepts

While the 3D DRAM and 3D resistive memory implementations in Section 1and Section 2 have been described with single crystal siliconconstructed with ion-cut technology, other options exist. One couldconstruct them with selective epi technology. Procedures for doing thesewill be clear to those skilled in the art.

Various layer transfer schemes described herein and in patent referenceincorporated can be utilized for constructing single-crystal siliconlayers for memory architectures described in Section 1, Section 2,Section 3 and Section 4.

FIG. 19A-B show it is not the only option for the architecture, asdepicted in FIG. 2-FIG. 18A-H, to have the peripheral transistors, suchas within bottom side periphery 1902, below the memory layers, such asmemory layer 1 1904, memory layer 2 1906, and memory layer 3 1908.Peripheral transistors, such as within topside periphery 1910, couldalso be constructed above the memory layers, as shown in FIG. 19B, andmay include substrate or memory layer 4 1912. This periphery layer wouldutilize technologies described in Section 1 and Section 2, and couldutilize junction-less transistors or recessed channel transistors, andmay utilize optical annealing, shielding layers, andabsorbers/reflectors as described in incorporated patent references.

The double gate devices shown in FIG. 2-FIG. 18A-H may have both gatesconnected to each other. Each gate terminal may be designed to becontrolled independently, which may lead to design advantages for memorychips.

One of the concerns with using n+ Silicon as a control line for 3Dmemory arrays is its high resistance. Using lithography and (single-stepof multi-step) ion-implantation, one could dope heavily the n+ siliconcontrol lines while not doping transistor gates, sources and drains inthe 3D memory array. This preferential doping may mitigate the concernof high resistance.

In many of the described 3D memory approaches, etching and filling highaspect ratio vias forms a serious limitation. One way to circumvent thisobstacle is by etching and filling vias from two sides of a wafer. Aprocedure for doing this is shown in FIG. 20A-E. Although FIG. 20A-Edescribe the process flow for a resistive memory implementation, similarprocesses can be used for DRAM, charge-trap memories and floating-gatememories as well. The process may include several steps that proceed inthe following sequence:

Step (A): 3D resistive memories may be constructed as shown in FIG.11A-K but with a bare silicon wafer 2002 instead of a wafer withperipheral circuits on it. Due to aspect ratio limitations, theresistance change memory and BL contact 2036 may be formed to the toplayers of the memory, as illustrated in FIG. 20A. Bit line contacts maybe formed, for example, as sidewall structures or end-wall structures,with various overlaps as required by the process and layout.Step (B): Hydrogen may be implanted into the wafer 2002 at a certaindepth such as hydrogen implant plane 2042. FIG. 20B illustrates thestructure after Step B including silicon oxide 2022.Step (C): The wafer with the structure after Step (B) may be bonded to abare silicon wafer 2044. Cleaving may be performed at the hydrogenimplant plane 2042. A CMP process may be conducted to polish off thesilicon wafer. FIG. 20C illustrates the structure after Step C.Step (D): Resistance change memory material and BL contact layers 2041may be constructed for the bottom memory layers. They may connect to thepartially made top BL contacts 2036 with state-of-the-art alignment. Bitline contacts may be formed, for example, as sidewall structures orend-wall structures, with various overlaps as required by the processand layout. FIG. 20D illustrates the structure after Step D, includinggate electrodes 2024, gate dielectric 2026, n+ silicon regions 2028, BLcontacts 2040, BLs 3038, WLs 2032, and SL 2034.Step (E): Peripheral transistors 2046 may be constructed usingprocedures shown previously in this document. FIG. 20E illustrates thestructure after Step E. Connections may be made to various wiringlayers.

The charge-trap and floating-gate architectures shown in FIG. 14A-F-FIG.18A-H are based on NAND flash memory. To one skilled in the art thatthese architectures can be modified into a NOR flash memory style aswell.

Section 6: Poly-Silicon-Based Implementation of Various Memory Concepts

The monolithic 3D integration concepts described herein can lead tonovel embodiments of poly-silicon-based memory architectures as well.Poly silicon based architectures could potentially be cheaper thansingle crystal silicon based architectures when a large number of memorylayers need to be constructed. While the below concepts are explained byusing resistive memory architectures as an example, it will be clear toone skilled in the art that similar concepts can be applied to NANDflash memory and DRAM architectures described previously in this patentapplication.

FIG. 21A-E shows an embodiment of the current invention, wherepolysilicon junctionless transistors are used to form a 3Dresistance-based memory. The utilized junction-less transistors can haveeither positive or negative threshold voltages. The process may includethe following steps as described in the following sequence:

Step (A): As illustrated in FIG. 21A, peripheral circuits 2102 may beconstructed above which a layer of silicon dioxide 2104 may be made.Step (B): As illustrated in FIG. 21B, multiple layers of n+ dopedamorphous silicon or polysilicon 2106 may be deposited with layers ofsilicon dioxide 2108 in between. The amorphous silicon or polysiliconlayers 2106 could be deposited using a chemical vapor depositionprocess, such as LPCVD or PECVD. The composition of the ‘SiO₂’ layerwithin the stacked Si/SiO₂ layers such as silicon dioxide 2108, may beinsulators or dielectrics other than silicon dioxide such as, forexample, a low-k dielectric, carbon containing silicon oxides, amorphouscarbon. The thickness of the ‘SiO₂’ insulator layer within the stackedSi/SiO₂ layers such as silicon dioxide 2108 may be adjusted to minimizelayer to layer, strata to strata disturb mechanisms, and may includethicknesses of 50 nm, 75 nm, 100 nm, 150 nm, 200 nm, 500 nm, and lessthan 1000 nm. A thin conductive layer, such as a metal, may also beformed between the two bonding oxides to form a field shield to mitigatelayer to layer, strata to strata disturb mechanisms, and may beelectrically floating or tied to a bias, such as ground or Vdd. Thestacked Si/SiO₂ layers may alternatively be formed by successive ionimplants of oxygen atoms/ions to various depths from the top surface ofa mono-crystalline silicon wafer/substrate and then heat treated to formoxide layers thus forming silicon layers in-between the oxide layers, alayered ‘SIMOX’ process approach.Step (C): As illustrated in FIG. 21C, a Rapid Thermal Anneal (RTA) maybe conducted to crystallize the layers of polysilicon or amorphoussilicon deposited in Step (B). Temperatures during this RTA could be ashigh as 210° C. or more, and could even be as high as 800° C. Thepolysilicon region obtained after Step (C) is indicated as 2110.Alternatively, a laser anneal could be conducted, either for all layers2106 at the same time or layer by layer.Step (D): As illustrated in FIG. 21D, procedures similar to thosedescribed in FIG. 10E-H may be utilized to construct the structureshown. The structure in FIG. 21D has multiple levels of junction-lesstransistor selectors for resistive memory devices, including isolationregions such as silicon oxide regions 2130. The resistance change memoryis indicated as 2136 while its electrode and contact to the BL isindicated as 2140. The WL is indicated as 2132, while the SL isindicated as 2134. Gate dielectric of the junction-less transistor isindicated as 2126 while the gate electrode of the junction-lesstransistor is indicated as 2124, this gate electrode also serves as partof the WL 2132. Gate dielectric 2126 and gate electrode 2124 may be, forexample, a HKMG structure or a TEL SPA radical oxidation oxide and anappropriate work function electrode, for example, tungsten, degeneratelydoped polysilicon or amorphous silicon. Although the width of the n+regions 2110 near and under the gates are illustrated as being largerthan the thickness, one skilled in the art would recognize that thereverse (i.e., thickness larger than width) could be formed to provideincreased gate control of the transistor channel.Step (E): As illustrated in FIG. 21E, bit lines (indicated as BL 2138)may be constructed. Contacts may be made to peripheral circuits andvarious parts of the memory array as described in embodiments describedpreviously.

FIG. 22A-F show another embodiment of the current invention, wherepolysilicon junction-less transistors are used to form a 3Dresistance-based memory. The utilized junction-less transistors can haveeither positive or negative threshold voltages. The process may includethe following steps:

Step (A): As illustrated in FIG. 22A, a layer of silicon dioxide 2204may be deposited or grown above a silicon substrate without circuits2202.Step (B): As illustrated in FIG. 22B, multiple layers of n+ dopedamorphous silicon or polysilicon 2206 may be deposited with layers ofsilicon dioxide 2208 in between. The amorphous silicon or polysiliconlayers 2206 could be deposited using a chemical vapor depositionprocess, such as LPCVD or PECVD described above. The composition of the‘SiO₂’ layer within the stacked Si/SiO₂ layers such as silicon dioxide2208, may be insulators or dielectrics other than silicon dioxide suchas, for example, a low-k dielectric, carbon containing silicon oxides,amorphous carbon. The thickness of the ‘SiO₂’ insulator layer within thestacked Si/SiO₂ layers such as silicon dioxide 2208 may be adjusted tominimize layer to layer, strata to strata disturb mechanisms, and mayinclude thicknesses of 50 nm, 75 nm, 100 nm, 150 nm, 200 nm, 500 nm, andless than 1000 nm. A thin conductive layer, such as a metal, may also beformed between the two bonding oxides to form a field shield to mitigatelayer to layer, strata to strata disturb mechanisms, and may beelectrically floating or tied to a bias, such as ground or Vdd. Thestacked Si/SiO₂ layers may alternatively be formed by successive ionimplants of oxygen atoms/ions to various depths from the top surface ofa mono-crystalline silicon wafer/substrate and then heat treated to formoxide layers thus forming silicon layers in-between the oxide layers, alayered ‘SIMOX’ process approach.Step (C): As illustrated in FIG. 22C, a Rapid Thermal Anneal (RTA) orstandard anneal may be conducted to crystallize the layers ofpolysilicon or amorphous silicon deposited in Step (B). Temperaturesduring this RTA could be as high as 700° C. or more, and could even beas high as 1400° C. The polysilicon region obtained after Step (C) isindicated as 2210. Since there are no circuits under these layers ofpolysilicon, very high temperatures (such as 1400° C.) can be used forthe anneal process, leading to very good quality polysilicon with fewgrain boundaries and very high mobilities approaching those of singlecrystal silicon. Alternatively, a laser anneal could be conducted,either for all layers 2206 at the same time or layer by layer atdifferent times.Step (D): This is illustrated in FIG. 22D. Procedures similar to thosedescribed in FIG. 10E-H may be utilized to get the structure shown inFIG. 22D that has multiple levels of junction-less transistor selectorsfor resistive memory devices, including insulator regions such assilicon oxide regions 2230. The resistance change memory is indicated as2236 while its electrode and contact to the BL is indicated as 2240. TheWL is indicated as 2232, while the SL is indicated as 2234. Gatedielectric of the junction-less transistor is indicated as 2226 whilethe gate electrode of the junction-less transistor is indicated as 2224,this gate electrode also serves as part of the WL 2232. Gate dielectric2226 and gate electrode 2224 may be, for example, a HKMG structure or aTEL SPA radical oxidation oxide and an appropriate work functionelectrode, for example, tungsten, degenerately doped polysilicon oramorphous silicon. Although the width of the n+ regions 2210 near andunder the gates are illustrated as being larger than the thickness, oneskilled in the art would recognize that the reverse (i.e., thicknesslarger than width) could be formed to provide increased gate control ofthe transistor channel.Step (E): This is illustrated in FIG. 22E. Bit lines (indicated as BL2238) may be constructed. Contacts may be made to peripheral circuitsand various parts of the memory array as described in embodimentsdescribed previously.Step (F): Using procedures described herein and in incorporated byreference patents, peripheral circuits 2298 (with transistors and wires)could be formed well aligned to the multiple memory layers shown in Step(E). For the periphery, one could use the process flow whereinreplacement gate processing is used, or one could use sub-400° C.processed transistors such as junction-less transistors or recessedchannel transistors. Alternatively, one could use laser anneals forperipheral transistors' source-drain processing. Connections can then beformed between the multiple memory layers and peripheral circuits. Byproper choice of materials for memory layer transistors and memory layerwires (e.g., by using tungsten and other materials that withstand hightemperature processing for wiring), or by using optical annealing andproper shielding layers, even standard transistors processed at hightemperatures (>1000° C.) for the periphery could be used.

Section 7: Monolithic 3D SRAM

The techniques described in this patent application can be used forconstructing monolithic 3D SRAMs.

FIG. 23A-D represents an SRAM embodiment of the current invention,wherein ion-cut is utilized for constructing a monolithic 3D SRAM.Peripheral circuits may be constructed on a silicon substrate, and abovethis, two layers of nMOS transistors and one layer of pMOS transistorsmay be formed using ion-cut and procedures described earlier in thispatent application. Implants for each of these layers may be performedwhen the layers are being constructed, and finally, after all layershave been constructed, a RTA may be conducted to activate dopants. Ifhigh k dielectrics are utilized for this process, a gate-first approachmay be preferred.

FIG. 23A shows a standard six-transistor SRAM cell according to anembodiment of the current invention. There are two pull-down nMOStransistors, and 2302 represents a pull-down nMOS transistor in FIG.23A-D. There are also two pull-up pMOS transistors, each of which isrepresented by 2316. There are two nMOS pass transistors 2304 connectingbit-line wiring 2312 and bit line complement wiring 2314 to the pull-uptransistors 2316 and pull-down transistors 2302, and these arerepresented by 2314. Gates of nMOS pass transistors 2314 are representedby 2306 and are connected to word-lines (WL) using WL contacts 2308.Supply voltage VDD is denoted as 2322 while ground voltage GND isdenoted as 2324. Nodes n1 and n2 within the SRAM cell are represented as2310.

FIG. 23B shows a top view of the SRAM according to an embodiment of theinvention. For the SRAM described in FIG. 23A-D, the bottom layer may bethe periphery. The nMOS pull-down transistors may be above the bottomlayer. The pMOS pull-up transistors may be above the nMOS pull-downtransistors. The nMOS pass transistors may be above the pMOS pull-uptransistors. The nMOS pass transistors 2304 on the topmost layer may bedisplayed in FIG. 23B. Gates 2306 for nMOS pass transistors 2304 arealso shown in FIG. 23B. Other numerals have been described previously inrespect of FIG. 23A.

FIG. 23C shows a cross-sectional view of the SRAM according anembodiment of the invention. Oxide isolation using a STI process isindicated as 2300. Gates for pull-up pMOS transistors are indicated as2318 while the vertical contact to the gate of the pull-up pMOS and nMOStransistors is indicated as 2320. The periphery layer is indicated as2398. Other numerals have been described in respect of FIG. 23A and FIG.23B.

FIG. 23D shows another cross-sectional view of the SRAM according to anembodiment of the current invention. The nodes n1 and n2 may beconnected to pull-up, pull-down and pass transistors by using a verticalvia 2310. 2326 is a heavily doped n+ Si region of the pull-downtransistor, 2328 is a heavily doped p+ Si region of the pull-uptransistor and 2330 is a heavily doped n+ region of a pass transistor.Other symbols have been described previously in respect of FIG. 23A,FIG. 23B and FIG. 23C. Wiring may connect together different elements ofthe SRAM as shown in FIG. 23A.

It can be seen that the SRAM cell shown in FIG. 23A-D is small in termsof footprint compared to a standard 6 transistor SRAM cell. Previouswork has suggested building six-transistor SRAMs with nMOS and pMOSdevices on different layers with layouts similar to the ones describedin FIG. 23A-D. These are described in “The revolutionary and truly3-dimensional 25F² SRAM technology with the smallest S³ (stackedsingle-crystal Si) cell, 0.16 um², and SSTFT (stacked single-crystalthin film transistor) for ultra high density SRAM,” VLSI Technology,2004. Digest of Technical Papers. 2004 Symposium on, vol., no., pp.228-229, 15-17 Jun. 2004 by Soon-Moon Jung; Jaehoon Jang; Wonseok Cho;Jaehwan Moon; Kunho Kwak; Bonghyun Choi; Byungjun Hwang; Hoon Lim;Jaehun Jeong; Jonghyuk Kim; Kinam Kim. However, these devices areconstructed using selective epi technology, which suffers from defectissues. These defects severely impact SRAM operation. The embodiment ofthis invention described in FIG. 23A-D is constructed with ion-cuttechnology and is thus far less prone to defect issues compared toselective epi technology.

It is clear to one skilled in the art that other techniques described inthis patent application, such as use of junction-less transistors orrecessed channel transistors, could be utilized to form the structuresshown in FIG. 23A-D. Alternative layouts for 3D stacked SRAM cells arepossible as well, where heavily doped silicon regions could be utilizedas GND, VDD, bit line wiring and bit line complement wiring. Forexample, the region 2326 (in FIG. 23D), instead of serving just as asource or drain of the pull-down transistor, could also run all alongthe length of the memory array and serve as a GND wiring line.Similarly, the heavily doped p+ Si region 2328 (in FIG. 23D), instead ofserving just as a source or drain of the pull-up transistor, could runall along the length of the memory array and serve as a VDD wiring line.The heavily doped n+ region 2330 could run all along the length of thememory array and serve as a bit line.

It will also be appreciated by persons of ordinary skill in the art thatthe invention is not limited to what has been particularly shown anddescribed hereinabove. For example, drawings or illustrations may notshow n or p wells for clarity in illustration. Moreover, transistorchannels illustrated or discussed herein may include dopedsemiconductors, but may instead include undoped semiconductor material.Further, any transferred layer or donor substrate or wafer preparationillustrated or discussed herein may include one or more undoped regionsor layers of semiconductor material. Moreover, although the insulatorbetween the stacked crystalline layers is identified as silicon oxide,other dielectrics may be utilized such as, for example, a low-kdielectric, carbon containing silicon oxides. Further, contacts may beformed, for example, as sidewall structures or end-wall structures, withvarious overlaps as required by the process and layout. Furthermore, thewiring of the peripheral circuits may be done using a lower meltingpoint metal than tungsten, for example copper, and care taken not toexceed a damaging temperature during processing and may employ opticalannealing. Rather, the scope of the invention includes both combinationsand sub-combinations of the various features described herein above aswell as modifications and variations which would occur to such skilledpersons upon reading the foregoing description. Thus the invention is tobe limited only by the appended claims.

1. A 3D semiconductor device, the device comprising: a first levelcomprising a single crystal layer, a plurality of first transistors andat least one metal layer, wherein said at least one metal layer overlayssaid single crystal layer and comprises interconnects between said firsttransistors forming control circuits; a second level overlaying said atleast one metal layer, said second level comprising a plurality ofsecond transistors; a third level overlaying said second level, saidthird level comprising a plurality of third transistors; and polysiliconpillars, wherein at least one of said second transistors comprises afirst channel region and at least one of said third transistorscomprises a second channel region, wherein a portion of at least one ofsaid polysilicon pillars is in direct contact with said first channelregion and said second channel region, wherein said second transistorsare aligned to said first transistor with less than 150 nm alignmenterror, wherein said second level and said third level thickness isgreater than 20 nm and less than 300 nm, wherein at least one of saidsecond transistors is self-aligned to at least one of said thirdtransistors, being processed following the same lithography step,wherein said second level comprises a plurality of first memory cells,said first memory cells each comprising at least one of said secondtransistors, wherein said third level comprises a plurality of secondmemory cells, said second memory cells each comprising at least one ofsaid third transistors, wherein at least one of said second memory cellsis at least partially atop of said control circuits, and wherein saidcontrol circuits are connected so to control at least one of said secondtransistors and at least one of said third transistors.
 2. The 3Dsemiconductor device according to claim 1, wherein said control circuitscomprise memory peripheral circuits comprising a plurality of decoders.3. The 3D semiconductor device according to claim 1, wherein said onemetal layer comprises tungsten, wherein the height distance from saidone metal layer to at least one of said first memory cells is less thanone micron, wherein said alignment error is less than 30 nm, and whereinsaid single crystal layer has a thickness greater than ten microns. 4.The 3D semiconductor device according to claim 1, wherein said secondtransistors are aligned to said first transistors with less than 150 nmmisalignment.
 5. The 3D semiconductor device according to claim 1,wherein said first memory cells are controlled thru a plurality ofmemory control lines, and wherein said third level comprises at leastone of said memory control lines.
 6. The 3D semiconductor deviceaccording to claim 1, wherein transport of control signals to said firstmemory cells and said second memory cells comprises a plurality ofword-lines and a plurality of bit-lines, and wherein said connectionbetween said word-lines and said control circuits comprises at least onestaircase.
 7. The 3D semiconductor device according to claim 1, furthercomprising: a fourth level overlaying said third level, wherein saidfourth level comprises a crystalline silicon layer.
 8. A 3Dsemiconductor device, the device comprising: a first level comprising asingle crystal layer, a plurality of first transistors and at least onemetal layer, wherein said at least one metal layer overlays said singlecrystal layer and comprises interconnects between said first transistorsforming control circuits; a second level overlaying said at least onemetal layer, said second level comprising a plurality of secondtransistors; a third level overlaying said second level, said thirdlevel comprising a plurality of third transistors; and polysiliconpillars, wherein at least one of said second transistors comprises afirst channel region and at least one of said third transistorscomprises a second channel region, wherein a portion of at least one ofsaid polysilicon pillars is in direct contact with said first channelregion and said second channel region, wherein said second transistorsare aligned to said first transistor with less than 150 nm alignmenterror, wherein said second level comprises a plurality of first memorycells, said first memory cells each comprising at least one of saidsecond transistors, wherein said third level comprises a plurality ofsecond memory cells, said second memory cells each comprising at leastone of said third transistors, and wherein said control circuits areconnected so to control at least one of said second transistors and atleast one of said third transistors.
 9. The 3D semiconductor deviceaccording to claim 8, wherein said control circuits comprises memoryperipheral circuits comprising a plurality of decoders, and wherein atleast one of said first memory cells at least partially overlays atleast one of said decoders.
 10. The 3D semiconductor device according toclaim 8, wherein said second transistors are aligned to said firsttransistors with less than 150 nm misalignment.
 11. The 3D semiconductordevice according to claim 8, wherein at least one of said secondtransistors is self-aligned to at least one of said third transistors,being processed following the same lithography step.
 12. The 3Dsemiconductor device according to claim 8, wherein transport of controlsignals to said first memory cells and said second memory cellscomprises a plurality of word-lines and a plurality of bit-lines, andwherein the connection between said word-lines and said control circuitscomprises at least one staircase.
 13. The 3D semiconductor deviceaccording to claim 8, further comprising: a fourth level overlaying saidthird level, wherein said fourth level comprises a crystalline siliconlayer.
 14. The 3D semiconductor device according to claim 8, whereinsaid one metal layer comprises tungsten, wherein the height distancefrom said one metal layer to at least one of said first memory cells isless than one micron, wherein said alignment error is less than 30 nm,and wherein said single crystal layer has a thickness greater than tenmicrons.
 15. A 3D semiconductor device, the device comprising: a firstlevel comprising a single crystal layer, a plurality of firsttransistors and at least one metal layer, wherein said at least onemetal layer overlays said single crystal layer and comprisesinterconnects between said first transistors forming control circuits; asecond level overlaying said at least one metal layer, said second levelcomprising a plurality of second transistors; a third level overlayingsaid second level, said third level comprising a plurality of thirdtransistors; and polysilicon pillars, wherein at least one of saidsecond transistors comprises a first channel region and at least one ofsaid third transistors comprises a second channel region, wherein aportion of at least one of said polysilicon pillars is in direct contactwith said first channel region and said second channel region, whereinsaid second transistors are aligned to said first transistor with lessthan 150 nm alignment error, and wherein said control circuits areconnected so to control at least one of said second transistors and atleast one of said third transistors.
 16. The 3D semiconductor deviceaccording to claim 15, wherein said second level comprises a pluralityof first memory cells, said first memory cells each comprising at leastone of said second transistors, and wherein said third level comprises aplurality of second memory cells, said second memory cells eachcomprising at least one of said third transistors.
 17. The 3Dsemiconductor device according to claim 15, wherein at least one of saidsecond transistors is self-aligned to at least one of said thirdtransistors, being processed following the same lithography step. 18.The 3D semiconductor device according to claim 15, wherein said onemetal layer comprise tungsten, wherein the height distance from said onemetal layer to at least one of said first memory cells is less than onemicron, wherein said alignment error is less than 30 nm, and whereinsaid single crystal layer has a thickness greater than ten microns. 19.The 3D semiconductor device according to claim 15, further comprising: astaircase structure, wherein said control circuits are connectedcomprises said staircase structure.
 20. The 3D semiconductor deviceaccording to claim 15, wherein fabrication processing of said devicecomprises a first processing of said first level followed by a secondprocessing of said second level on top of said first level, and thenprocessing said third level on top of said second level, and whereinsaid first processing said first level accounts for the temperatureassociated with processing said second level and said third level byadjusting the process thermal budget of said first level accordingly.